Factor out x^2-3x-40. Know that the zeroes of a function multiply to be equal to the constant (-40) and add up to the coefficient of the x term (-3). The two numbers that do that are -5 and 8. Recall that to be zeroes, at least one of the x-values must make the equation equal 0. So if x=-5, (x+5) is the term that would be 0.
You could also multiply the factored forms that are given to find the correct one.
