Hello, everyone, I have a question about using standard deviation to explain changes in the dependent variable when I review relevant empirical microeconomic literature.

Suppose the estimated regression model is as follows:
z=beta_1*x+beta_2*y+epsilon

The literature which I refer comes out with the conclusion that a standard deviation of (beta_1*x+beta_2*y) explains 15% of the standard deviation of z.

How can I obtain the value 15% here if I know the standard deviations of x, y, and z?

Your comments are appreciated and I'm grateful for your suggestions.