First look at the value before the squared variable. Since this is 1, We know that both will have 1. (a+?)(a-?)
We know that the there will be a plus and minis sign because the bx value is positive while the c value is negative.
Now find the factors for the c value (-40). 10 and 4, 5 and 8
10 and 4 can be subtracted to get the bx value of 6. But not 5 and 8
Since the bx value is positive, the bigger number has to be positive.
So your answer is (a+10) (a-4)
A quick check gives you a2+10a - 4a - 40
Combine like terms to get the oronal problem.