Solve the equation by factoring. 9 xsquared +21x -8= 0

9x² + 21x - 8 = 0. This equation could be written as a(x-x')(x-x"), where 
a =9, x' = the 1st root and x" = the 2nd root..
Find the roots:

x'= [-b+√(b²-4.a.c)]/2.a   and  x"= [-b-√(b²-4.a.c)]/2.a

x' = 1/3  and x" = -8/3 and the initial factorized equation becomes:

9(x-1/3)(x+8/3)

After reduction to same denominator and simplifying by 9, you get:
(3x-1)(3x+8) ( = 9x² + 21x - 8)