the first step to solve this question is removing the (), it's way easier to solve a problem knowing what the question really says.
(2x²+3)(4x+5)-8x(x²-2)
= 2x²×4x + 2x²×5 + 3×4x + 3×5 -8x×x² -8x×-2
= 8x³+10x²+12x+15-8x³+16x
now, we add up the numbers with the same x's
8x³+10x²+12x+15-8x³+16x
= 0x³+10x²+28x+15
=10x²+28x+15
