If $X$ is a random variable, then $E(X + k) = E(X) + k,$ $E(cX) = c E(X),$
$Var(X + k) = Var(X),$ and $Var(cX) = c^2 Var(X).$
(These are facts for any random variable, including normal variables.)
If you already know these facts, you can use them to determine the mean and variance
of $Y - \mu,$ then use them again to find your desired result.
If $X$ is a random variable, then $E(X + k) = E(X) + k,$ $E(cX) = c E(X),$ $Var(X + k) = Var(X),$ and $Var(cX) = c^2 Var(X).$ (These are facts for any random variable, including normal variables.)
If you already know these facts, you can use them to determine the mean and variance of $Y - \mu,$ then use them again to find your desired result.