given f(x) = x^2+3 and g(x) = 2x+a and fg(x) = 4x^2-8x+7 calculate the value of the constant a
well,
(fg)(x) = f(x)*g(x)
= (x^2+3)(2x+a)
expand that out and equate it to 4x^2-8x+7. The coefficients must all match. That will give you the value of a.
Better check for typos, since fg = 2x^2 + ... and there's no way to match that with 4x^2+...