Find f(x) x g(x) if f(x)b= 8x+1 and g(x) =-5x+9

It's best not to use " X " to denote mult.  Use " * " for that purpose.

f(x) X g(x) if f(x)b= 8x+1 and g(x) =-5x+9 should be:

f(x) * g(x) if f(x)b= 8x+1 and g(x) =-5x+9

I don't understand what you mean when you include " b " in this problem.  I will temporarily ignore the " b " and focus on solving:

f(x) * g(x) if f(x)= 8x+1 and g(x) =-5x+9

(f * g)(x) = (8x+1)(-5x+9) = -40x^2 + 72x - 5x + 9 = -40x^2 + 67x + 9.