How do you do systems of substitution ?

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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.

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