a(x^2+x-c)+b(2x^2-x-2)
=ax? +ax-ac+2bx? -bx-2b
=(a+2b)x? +(a-b)x-ac-2b=7x? +4x+3
therefore
a+2b=7
a-b=4
-ac-2b=3
Simultaneous solution: a=5, b= 1, c=- 1.
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