If x = -13, just plug -13 in for x in the equation, simplify, isolate the variable, and solve.
-4x + 5y = 12
-4(-13) + 5y = 12
-4 • -13 = 52
52 + 5y = 12
12 - 52 = -40
5y = -40
-40 / 5 = -8
y = -8
So when x is -13, y is -8.
Check the solution:
-4x + 5y = 12
-4(-13) + 5(-8) = 12
52 + (-40) = 12
52 - 40 = 12
12 = 12
It works, so this is correct.
