Sum of roots of a functional equation
If $x$ is a root then so is $58-x$ because $f(58-x)=f(x)$. Hence, the three roots are of the form $x, 58-x,29$ with $ x\neq 29$. The sum is $87$.
Sum of roots of a functional equation
If $x$ is a root then so is $58-x$ because $f(58-x)=f(x)$. Hence, the three roots are of the form $x, 58-x,29$ with $ x\neq 29$. The sum is $87$.