The variances are \(\operatorname{Var}(X)=3^2=9\) and \(\operatorname{Var}(Y)=4\).

(a) Independence gives \(\operatorname{Var}(X+Y)=9+4=13\). Hence the standard deviation is \(\sqrt{13}=3.61\) to 3 significant figures.

(b) For independent variables, \[\operatorname{Var}(5X-Y)=5^2\operatorname{Var}(X)+(-1)^2\operatorname{Var}(Y)=25(9)+4=229.\] The standard deviation is \(\sqrt{229}=15.1\) to 3 significant figures.