Add to both sides of what? So far you don't have an equation. Please show exactly what you did. When I use "complete the square", I get the correct answer.
When I try to solve x^{2}+3x-10 just by factoring it from it's standard form I get x=-5 and x=2 which is good. But when I try to check that by using the "completing the square method" when you add (b/2)^{2 }to both sides I get x=5/6 which is completely different as you can see. It is said that the "completing the square method works for any problem.