System of equations is like subtracting and substituting two or more equations! For example, if there are two equations:
2x+5y=19
-2x+3y=2
You would subtract the two equations straight across to eliminate one of the variables (either x or y). Because -2 and 2 are opposites of each other, if you add them, they equal zero, which leads to you eliminating a variable! Then, you would add 5y+3y=8y and 19+2=21 and then do 8y=21 and you get your answer for y, which in this case is 21/8.
If you want to find the x value next, you would plug in your y value into the equation and solve for x! Example:
2x+5(21/8)=19!