Use synthetic division to divide.
(4x3 + 26x2 + 10x - 12) ÷ (x + 6)
"spacing" does not work too well on this board, so please adjust the spacing ...
-6 │ 4 26 10 -12
-----------------
4 2 -2 0
so (4x^3 + 26x^2 + 10x - 12) ÷ (x + 6)
= 4x^2 + 2x - 2