a. The average value of [tex]f[/tex] on the given interval is
[tex]\displaystyle f_{\rm ave}=\frac1{\pi-0}\int_0^\pi(2\sin x-\sin2x)\,\mathrm dx=\boxed{\frac4\pi}[/tex]
b. Solve for [tex]c[/tex]:
[tex]\dfrac4\pi=2\sin c-\sin2c\implies\boxed{c\approx1.238\text{ or }c\approx2.808}[/tex]