The function f is defined by \(\mathrm{f}(x)=15 \cos ^{2}(3 x+1.5)+7 \sin (3 x+1.5)-13\) for \(-0.3 \leqslant x \leqslant 0.5\), where \(x\) is in radians. (a) Solve the equation \(\mathrm{f}(x)=0\).

(b) Find the \(x\)-coordinates of the two stationary points on the curve \(y=\mathrm{f}(x)\).