This is unclear. You are finding the sqrt of what? of 3x alone? of 3x+28? Please present the question here exactly as you found it.
If you meant sqrt(3x+28) - 8 = x, then sqrt(3x+28)=x+8.
Squaring both sides, 3x+28 = x^2 + 16x + 64, which simplifies to
x^2 + 16x - 3x + 64 - 28 = 0, or x^2 + 13x + 36 = 0.
Note that 9 and -4 multiplied together produce -36, which has the wrong sign here.
Again I must ask you to verify that you have copied down this problem completely and correctly.