Question

Factorise x^2 + 3x -40

Answer 1

Notice, all we do is "prime factor" 40, and use the factors about to add them and get the middle coefficient of 3, and since they're factors, their product will give us 40 back.

$\bf x^2\stackrel{8-5}{+3}x\stackrel{(8)(-5)}{-40}\implies (x+8)(x-5)$