# Find the PDF of Z where $Z = X^2 + Y^2$ [duplicate]

$X$ and $Y$ are independent normal variable with zero mean and common variance $k$, Find the Probability Density of Function of $Z = X^2 + Y^2$? We can although find the PDF of a random variable if it is function of one random variable with know PDF, but how to calculate for $2$ variables. I was not able to proceed. How to use the given data of common variance?