how to factorize? thanks?

5(3x+y)^2+6(3x+y)-8
let u=3x+y
factors of -40 that add to 6: 10 & -4

5u^2+6u-8
5u^2 + 10u - 4u - 8
5u(u+2) - 4(u+2)
(5u-4)(u+2)
(5(3x+y)-4)(3x+y+2)
(15x+5y-4)(3x+y+2)

This is a quadratic in (3x + y).

So let u = 3x + y, and you get

5u^2 + 6u - 8. Then just factor

that, then substitute back.