Question

(x+6/x^2+8x+15)+(3x/x+5)-(x-3/x+3)

Answer 1

Please post what you want to solve in addition to the problem in the future.

Assuming you want to simplify:
(x + 6)/((x+3)(x +5) + (3x/x-5) - (x-3/x+3)
Make all the parts have like denominators (aka make everything have a denominator of (x+3)(x+5)).

(x+6)+(3x)(x+3)-(x-3)(x+5)/
((x+3)(x+5))

(x+6 + 3x^2 + 9x - (x^2 + 2x - 15))/
((x+3)(x+5)

(x + 6 + 3x^2 + 9x - x^2 - 2x + 15)/
((x+3)(x+5))

(2x^2 + 8x + 21)/((x+3)(x+5)) is the final fraction.