We need two numbers that when multiplied will give you -40 and when added will give you -3:
(x+5) (x-8) this is your answer. And you want to know the process Here it goes. And attached is a chart I made you so you can visually see what this process is telling you:
1) identify a,b, and c in the trinomial ax^2 + bx+c (in your case it's 1x^2-3x-40= x^2-3x-40)
2) write down all factor pairs of c (numbers that multiplied produce -40)
3) identify which factor pair from the previous step sums up to b
4) Substitute factor pairs into two binomials