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.

The pie is multyplued by the second step

Where is your question?