Question 600799
-4c^2+19c-21

a= -4, b= 19, c= -21 

ac= (-4*-8)= +84

factorise 84 
84= 2*2*3*7

Try  a combination of these factors  to add up to b=19

2*2*3= 12 
& 7
12 & 7 are the factors

-4c^2+12c+7c-21=0

-4c(c-3)+7(c-3)
(c-3)(7-4c)