 We're going to talk about substitution, this is an introduction to substitution. The first thing that we need to do is just to practice the skills that we'll need. We're just trying to find y when x equal 4. x is equal to a number 4, so everywhere we see x, we're going to plug in 4. So 9 times y plus it's 3 times x, so now it's 3 times 4. It's equal to 2 times x, so it's 2 times 4 minus 8. So 9y plus 12 is equal to 8 minus 8, and 9y plus 12 on this side is equal to 0. If we subtract 12 from both sides, it'll be 9y equal negative 12 divided by 9. y is equal to negative 12 over 9, or if we reduce it, both visible by 3, it's negative 4 over 3. In this case, we're to find x and y is equal to an expression. So everywhere we see y, which happens to be right here, we're going to replace it with 3x minus 5. Negative 2x plus 5 times 3x minus 5 is equal to 1. So we have to distribute the 5 and combine like terms. So first distribute negative 2x plus 15x minus 25 is equal to 1. Combine like terms, we're going to have 13x minus 25 is equal to 1. And if we add 25 to both sides, that leaves 13x on the left hand side equal to 26. And when you divide by 13, x is going to be equal to 2. This time, we're going to find y when x is equal to an expression. So we're going to replace x anywhere we see it. So negative 5 times that for y plus 4 plus 3y is equal to 14. Again, we have to distribute because we're multiplying by an expression. So negative 20y minus 20 plus 3y is equal to 14. Combining our like terms, we have negative 17y minus 20 is equal to 14. If we add 20 to both sides, 20 and 14 would be positive 34 divided by negative 17. And y will be equal to negative 2. Now, when we solve a system with substitution, we're going to isolate on one side a variable in either equation, doesn't matter which one. Then we're going to plug that into the other equation solve. And then we also have to find the other variable. It's an x and a y solution. So here we have y equal 5x. It's already isolated, so number one's done for us. Now we just have to plug that in for y. So we have 2x plus y, but y is no longer y, it's now 5x equal to 7. So 2x plus 5x is 7x equal to 7, and x then would be equal to 1. So now we know x equal 1, but we don't know y. But right here, we know that y is equal to 5 times whatever x is. So y is going to be equal to 5 times 1 or y equal 5. In the next video, we'll try some other examples.