$($b$)$ Suppose that $f(x,y)=5ysin(3x)$ where $x(r,s)=3r-s,y(r,s)=r{e}^{2s}$ and $s(t)=t^2+1.$ Use chain rule to find the value of $\frac{\text{d}\text{e}\text{l}\text{f}}{\text{d}\text{e}\text{l}\text{t}}$ when $r=,s=0$ and $t=2.$