x+15% is probably the same as x+(15% of x), that is, x+0.15x or x(1+0.15)=1.15x, making the equation 1.15x=76.77.
So x=76.77/1.15=66.76 approx.
The only other interpretation is to convert 15% into a decimal 0.15: x+0.15=76.77 so x=76.62, but I don’t think that’s how the question should be interpreted because it has no practical meaning.