a) $\left\{\begin{matrix}y= -x^{2}+6x-5 & & & \\ y=-x^{2}+4x-3& & & \\ y=3x-15& & & \end{matrix}\right.$
b) $\left\{\begin{matrix} y=\left | x^{2}-4x \right | & \\ y=\left | 2x-7 \right | +1& \\ x=-1 & \\ x=2 \end{matrix}\right.$
c) $\left\{\begin{matrix} y=sin^{2}x.cos^{3}x\\ y=0\\ x=0\\ x=\frac{\pi }{2} \end{matrix}\right.$