Answer:
[tex]\large\boxed{r=\dfrac{3A-hb}{h}}[/tex]
Step-by-step explanation:
[tex]A=\dfrac{1}{3}h(r+b)\qquad\text{multiply both sides by 3}\\\\3A=h(r+b)\qquad\text{use distributive property}\\\\3A=hr+hb\qquad\text{subtract hb from both sides}\\\\3A-hb=hr\qquad\text{divide both sides by }\ h\neq0\\\\\dfrac{3A-hb}{h}=r[/tex]
