OK, you left some parentheses out, but I think the equation is:
(4n+5)/(n+3) - 1/(n+4) = 1/(n2+7n+12)
Note that n2+7n+12 can be factored and equals (n+4)(n+3):
(4n+5)/(n+3) - 1/(n+4) = 1/(n+4)(n+3)
Multiply both sides by (n+4)(n+3):
(4n+5)(n+4) - (n+3) = 1
4n2 + 16n + 5n + 20 - n -3 -1 = 0
4n2 + 20n + 16 = 0
4(n2 + 5n + 4) = 0
Can you finish it from here? The quadratic in the parentheses can be factored, or you can always use the quadratic formula.