To make x the subject of this equation, you need to set the equation equal to x. Or in other words, get the x variable all on its own on one side of the equation. To do this, simply add 5 to both sides of the equation.
The left hand side of the equation will be b+5
And the right hand side of the equation will be just x because -5+5=0 leaving just the x.
b+5=x or x=b+5
That is your answer.