Simplify the sum.
(d^2 - 7d + 12)/(d^2 - d - 6) + (d^2 + 5d + 6)/(d^2 - 2d - 15)
The sum is:
((d-4)(d-3))/(d-3)(d+2) + ((d+3)(d+2))/(d-5)(d+3)
Simplify by canceling out the common terms:
= (d-4)/(d+2) + 1/(d-5)
= (d-4 + (d+2))/(d+2)(d-5)
= (2d - 2) / (d+2)(d-5)
= 2(d-1) / (d+2)(d-5)