so heres the question

x^4 + 3x^3 -5x^2 -19x -60 ≤ 0

i did synthetic division with 3 and got x^3 + 6x^2 +13x +20 =0

then i did it again with -4 and got x^2 +2x +5

so generally i have these so far

(x-3) (x+4) (x^2+2x+5)

im stuck on the third term

where do i go from here?