You are looking for an integer, x, such that
xsol = solve(131-x^2 == 20)
You can see from xsol that the x that solve that equation are not integers: they are numbers that are between 10 and 11 and the negative of that.
Changing to a smaller increment such as 0.1 will only get you closer . No matter what rational increment you use instead of 1, the actual solutions are irrational and so cannot be reached by the method you are using (though possibly you could find something that came out okay to within roundoff error