Okay so we're given:
Area = x^2 + 5x -84
width = x +12
We need to find length, so for now, let's say length = L
We know Area = Length x Width so filling what we have into this equation...
Area = Length*width
x^2 +5x -84 = L*(x+12)
Divide both sides by (x +12) and you get...
(x^2 +5x-84)/(x+12) = L
Do that division and you get
L = x-7
Let me know if you need help with the polynomial division, there are lots of good videos on it on youtube!