Step-by-step explanation:
[tex]a)\\A=\dfrac{1}{2}h(a+b)\qquad\text{multiply both sides by 2}\\\\2A=2\!\!\!\!\diagup\cdot\dfrac{1}{2\!\!\!\!\diagup}h(a+b)\qquad\text{use the distrigutive property}\\\\2A=ha+hb\qquad\text{subtract}\ hb\ \text{from both sides}\\\\2A-hb=ha\qquad\text{divide both sides by}\ h\neq0\\\\\dfrac{2A-hb}{h}=a\to \boxed{a=\dfrac{2A-hb}{h}}[/tex]
[tex]b)\\a=\dfrac{2A-hb}{h}\\\\\text{Substitute:}\ A=19.95\ cm^2,\ b=7.8\ cm,\ h=3.5\ cm\\\\a=\dfrac{2(19.95)-(3.5)(7.8)}{3.5}=\dfrac{39.9-27.3}{3.5}=\dfrac{12.6}{3.5}=3.6\\\\\boxed{a=3.6\ cm}[/tex]
