Using the splitting middle term - the middle term of the general equation is divided in two such values that:

Product = a.c

For the given equation a = 15 ; b = -1; c = -28

= 15. - 28 = - 420

And either of their sum or difference = b

= - 1

Thus the two terms are - 21 and 20

Difference = - 21 + 20 = - 1

Product = - 21.20 = - 420

3x(5x-7) + 4(5x-7) = 0

(5x-7)(3x + 4) = 0

(5x-7) = 0 or (3x + 4) = 0

x = 7/5 or x = (-4)/3

Roots of equation are 7/5, - 4/3