How do you do systems of substitution ?

You use it when you are given 2 equations such as,

x + y = 12

y= 3x

Since you are given the value of y (3x), you can substitute that value into the second equation.

ie..,

x+y=12
x+(3x)=12
4x=12
4x/4=12/4
x=3

With the newly solved for value of x, you can sub it into one of the equations to solve for y.

x + y = 12

(3) + y = 12

y = 12-3

y = 9

So the key point is, when you are given the value of one variable sub it into the second equation! 

Solve one of the equations for either x or y and then substitute it into the other equation to solve for the variable.
When you find the one value plug it back in to find the other variables value.