The method to solve this answer is fairly simple.
First, write out your brackets like so: (x )(x ), leaving a space for the number that you will use in this factorisation
Next, find two numbers that multiply to give you +20 (the integer), but also add to give you -9 (the coefficient of x). The two numbers are -4 and -5
Now, you can place the two numbers into the brackets to give you the factored form:
(x-4)(x-5)
We know this is correct because if you expand these brackets, you get your original expression (shown in the attached image)