It's just an equation, the number you're looking for is X, and then turn the words into an solvable equation with respect to X.
7X + 3 = 39 - 2X
9X = 39 -3 = 36
X = 4
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The answer is going to be
X=20
Hope this helps!
the equation is y=mx+b and when you plug in the numbers it would be y=2x+3