[tex]A(b)=10\cdot\dfrac{b+7}{2}=5(b+7)=5b+(5)(7)=5b+35\\\\a=5b+35\qquad\text{solve for b}\\\\5b+35=a\qquad\text{subtract 35 from both sides}\\\\5b=a-35\qquad\text{divide both sides by 5}\\\\b=\dfrac{a}{5}-\dfrac{35}{5}\\\\b=\dfrac{a}{5}-7\\\\Answer:\ \boxed{D.\ B(a)=\dfrac{a}{5}-7}[/tex]
