SOLUTION: I can not understand how to factor trinomials in ax^2+bx+c format. I have been studying this for two weeks at school now. I understand the process, but I for some reason am horribl

Question 600799: I can not understand how to factor trinomials in ax^2+bx+c format. I have been studying this for two weeks at school now. I understand the process, but I for some reason am horrible at this. Here is a problem I need help solving.
-4c^2+19c-21
I need to find the two numbers that equal the variable.
Answer by mananth(16946) (Show Source): You can put this solution on YOUR website!
-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)
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