Let the output = Y and the input = X.
The output (y) is 7 more than 3 times the input (x), which can be written as an equation:
Y = 3X+7
The out put is -8, so replace Y in the equation and solve for X:
-8 = 3X+7
Subtract 7 from each side:
-15 = 3X
Divide both sides by 3:
X = -15 / 3
X = -5
The input is -5