Answer:
-5
Explanation:
Substituting x=4 into the equation gives a 2-step linear equation in y. It is solved by isolating the variable and making its coefficient be 1.
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use x=4
When x=4, the equation becomes ...
-3x +9y = -57
-3(4) +9y = -57
-12 +9y = -57
solve 2-step equation
The first step is to "isolate" the variable term (9y) by adding the opposite of the constant that is on the same side of the equation. The result is that the variable term is by itself on one side of the equal sign.
-12 +12 +9y = -57 +12 . . . . . add the opposite of -12
9y = -45 . . . . . . . . . . . . . . simplify
The second step is to make the coefficient of y be 1. We do that by multiplying by its inverse, 1/9. Equivalently, we divide by 9.
(1/9)(9y) = (1/9)(-45) . . . . multiply by the inverse of 9
y = -5 . . . . . . simplify