$6-21.$ The compound beam is subjected to a uniform dead load of $200l\frac{b}{f}t$ and a uniform live load of $150l\frac{b}{f}t.$ Determine $($a$)$ the maximum negative moment these loads develop at $A,$ and $($b$)$ the maximum positive shear at $D.$ Assume $B$ is a pin and $C$ is a roller. SOLVE USING EQUATIONS AND EXPLAIN HOW MOMENT IS $0$ AT A PLEASE