[tex]\displaystyle\bf\\7)\\\\\\y=\frac{ab}{2c} \\\\\\2c=\frac{ab}{y} \\\\\\\boxed{c=\frac{ab}{2y} }\\\\\\8)\\\\\\\frac{a^{2} +4a+4}{3a-9} :\frac{a+2}{a-3} =\frac{(a+2)^{2} }{3\cdot(a-3)} \cdot\frac{a-3}{a+2} =\boxed{\frac{a+2}{3} }\\\\\\9)\\\\\\\frac{x^{2} -2x}{3x+3} \cdot\frac{x+1}{x-2} =\frac{x\cdot(x-2)}{3\cdot(x+1)} \cdot\frac{x+1}{x-2} =\boxed{\frac{x}{3} }[/tex]