Find the value of x and if necessary, express answer in simplest radical form.

Writing and solving an equation of ratios is the way to go here.  See if you can figure out why the following is correct:

x+3      x
=
  12     x+3

Then (x+3)^2 = 12x, and x^2 + 6x + 9 = 12x, so that x^2 - 6x + 9 = 0

This is the square of (x - 3).  Thus, x-3 = 0, and x = 3.

The value of x is 3.

Ithink it's b sorry if i'm wrong