Answer:
(7x−5)(3x+4)
Explanation:
Step-1 : Multiply the coefficient of the first term by the constant -21 • 20 = -420
Step-2 : Find two factors of -420 whose sum equals the coefficient of the middle term, which is 13 .
-420 + 1 = -419
-210 + 2 = -208
-140 + 3 = -137
-105 + 4 = -101
-84 + 5 = -79
-70 + 6 = -64
-60 + 7 = -53
-42 + 10 = -32
-35 + 12 = -23
-30 + 14 = -16
-28 + 15 = -13
-21 + 20 = -1
-20 + 21 = 1
-15 + 28 = 13 That's it
Step-3 : Rewrite the polynomial splitting the middle term using the two factors found in step 2 above, -15 and 28
-21x2 - 15x + 28x + 20
Step-4 : Add up the first 2 terms, pulling out like factors :
-3x • (7x+5)
Add up the last 2 terms, pulling out common factors :
4 • (7x+5)
Step-5 : Add up the four terms of step 4 :
(-3x+4) • (7x+5)