OK, let's first subtract 5 from both sides
This gives SQRT(3-x) = 4
Now, to get rid of the root, square both sides
We have 3-x = 16
Subtract 3 from both sides
This gives -x = 13
Multiply through by -1
We get x = -13
You should check for yourself that this is correct by putting the answer back into the original problem to make sure it makes sense.
I'll leave that for you........