Answer is A.
You start by adding the 7 to the opposite side. Giving you 19. You divide both sides by -2 which gives you x on one side and -9.5 on the other.
-2x-7=12
-2x=12(+7)
-2x=19
x=19/-2
x=-9.5
The greatest number of sets would be 20 with one girl and one boy in each.
Dy/dx = (dy/dt) / (dx/dt)
dy/dt = 2bt
dx/dt = 3at^2
therefore dy/dx = 2bt/3at^2
dy/dx = 2b/3at
It is a similar triangle so the side length of the smaller triangle (x and x-2) are in the same ratio as the bigger triangle (2x+5 and 2x-1). As the ratio is the same, you can equate them and solve for x.