The thing with polynomials is, if you have two of the same polynomial, then all of their coefficients are the same. Meaning, if you have

f1(x) = c1*x^2+b1*x+a1

f2(x) = c2*x^2+b2*x+a2

f1(x)=f2(x)

then necessarily,

a1=a2, b1=b2, c1=c2

In other words, you can't find some other set of (a, b, c) which are different, but produce the same function. So specifically, in your case, you have one polynomial on the left side (9x^2 +42x+49) and another one on the right side (a+3bx-2cx^2 ).

Ask yourself which coefficient on the left side is equivalent to which coefficient on the right side.