This "substitution" method starts out by "solving for x" (or any other variable) in one of the equations. For example, let's say your equations are 4x + 2y = 8 and 5x + 3y = 9. Start by looking just at the first equation. Rearrange it by subtracting 2y from each side, to get: 4x = 8 - 2y. This method often uses fractions later on. You can try the elimination method below instead if you don't like fractions. " Once you have the x term (or whichever variable you are using) on one side of the equation, divide both sides of the equation to get the variable alone. For example:  4x = 8 - 2y (4x)/4 = (8/4) - (2y/4) x = 2 - ½y Make sure you go back to the other equation, not the one you've already used. In that equation, replace the variable you solved for so only one variable is left. For example:  You know that x = 2 - ½y. Your second equation, that you haven't yet altered, is 5x + 3y = 9. In the second equation, replace x with "2 - ½y": 5(2 - ½y) + 3y = 9. You know have an equation with only one variable. Use ordinary algebra techniques to solve for that variable. If your variables cancel out, skip ahead to the last step. Otherwise, you'll end up with an answer for one of your variables:  5(2 - ½y) + 3y = 9 10 – (5/2)y + 3y = 9  10 – (5/2)y + (6/2)y = 9 (If you don't understand this step, learn how to add fractions. This is often, but not always, necessary for this method.) 10 + ½y = 9 ½y = -1 y = -2 Don't make the mistake of leaving the problem half-finished. You'll need to plug the answer you got back into one of the original equations, so you can solve for the other variable:  You know that y = -2  One of the original equations is 4x + 2y = 8. (You can use either equation for this step.) Plug in -2 instead of y: 4x + 2(-2) = 8. 4x - 4 = 8 4x = 12 x = 3 When you plug x=3y+2 or a similar answer into the other equation, you're trying to get an equation with only one variable. Sometimes, you end up with an equation with no variables instead. Double check your work, and make sure you are plugging the (rearranged)...

TL;DR in Vietnamese:
Chuyển vế các biến số trong phương trình. Chia cả hai vế của phương trình để "giải tìm x. Thay kết quả này vào phương trình còn lại. Giải tìm biến số còn lại. Sử dụng đáp án này để tìm biến số còn lại. Khi hai biến số triệt tiêu thì phải làm gì?