Explanation:-
First Method: (By splitting the middle term)
We have to find p and q such that p+q = 20 and
pq = 3×(-7) = -21
Let us looks for the pair of factors of -21 such that their sum is 20 and product is -21.
20 = 21 - 1 and -21 = 3(-7)
∴ 3x² + 20x - 7 = 3x² + 21x - 1x - 7
= 3x(x + 7) -1(x + 7)
= (x + 7)(3x - 1)
Second method: (Using the factor theorem)