Ineed to know how to do the systems of equation

x = 3
              y = -1

There are many ways to do the systems of equations. To solve this, I used the substitution method.

Isolate y or x (whichever is easier) in the first equation IF NECESSARY.
y = x - 4 --> it's already given

Substitute in (x - 4) for y in the second equation.
2x + (x - 4) = 5                 Distributive Property
2x + x - 4 = 5                   Combine like terms
3x - 4 = 5                         Add 4 to both sides
3x = 9                              Divide by 3 to both sides
x = 3                                Answer!

Substitute in (3) for x in the first equation.
y = (3) - 4                        Simplify
y = -1                               Answer!

196 cu ft