Solve the system of equations.

-4x+7y=20
y=3x+15

Solve by Substitution :

// Solve equation [2] for the variable y

[2] 5y = 15x - 4

[2] y = 3x - 4/5

// Plug this in for variable y in equation [1]

[1] 15x - 20•(3x-4/5) = 7

[1] -45x = -9

// Solve equation [1] for the variable x

[1] 45x = 9

[1] x = 1/5

// By now we know this much :

x = 1/5

y = 3x-4/5

// Use the x value to solve for y

y = 3(1/5)-4/5 = -1/5

Solution :

{x,y} = {1/5,-1/5}