(a^2 - 4a)/(a^2-a-6) minus (a-6)/a^2-a-6)

combine terms to get

(a^2 - 4a - a + 6)/(a^2-a-6)
= (a^2-5a+6)/(a^2-a-6)
= (a-3)(a-2)/((a-3)(a+2))
= (a-2)/(a+2)
if a≠3