r² + 7 r + 10 = r² + 2 r + 5 r + 10 = r ( r + 2 ) + 5 ( r + 2 ) = ( r + 2 ) ( r + 5 )
r² - 5 r - 50 = r² - 10 r + 5 r - 50 = r ( r - 10 ) + 5 ( r - 10 ) = ( r - 10 ) ( r + 5 )
[tex] \frac{r^{2} +7r+10}{3}* \frac{3r-30}{r ^{2}-5r-50 }= \\ \frac{(r+5)(r+2)}{3}* \frac{3(r-10)}{(r-10)(r+5)} [/tex] = ( after cancellation ) = r + 2
