Question

(3+sqrt x)(4- 3sqrt x)

is the answer 12- 5x sqrt

Answer 1

You forgot to multiply sqrt(x) with -3sqrt(x) and add this to the result.

If you do so, you get an additional term with -3x

Answer 2

So it would be 12-5sqrtx-3x?

Answer 3

Yes, that's the correct answer.