For quadratics with an a value (coefficient of x²) of 1, we find factors of c, the constant, that sum to b, the coefficient of x. In our case we want factors of -16 that sum to 6.
-16 = -1(16) or 16(-1)
-16 = -2(8) or 8(-2)
-16 = -4(4) or 4(-4)
Out of these 6 possibilities, the only one that sums to 6 is 8(-2), because 8+(-2)=6.
This gives us the factors (x+8)(x-2).