We need to isolate the dependent variable $y$, we can do that by simultaneously subtracting $5x^2\cos\left(x\right)$ from both sides of the equation

Removing the variable's exponent

Cancel exponents $2$ and $1$

As in the equation we have the sign $\pm$, this produces two identical equations that differ in the sign of the term $\sqrt{1-5x^2\cos\left(x\right)}$. We write and solve both equations, one taking the positive sign, and the other taking the negative sign

Combining all solutions, the $2$ solutions of the equation are