$7k \times \left( {k}^{2}-2k+7 \right)-3\left( {k}^{2}-2k+7 \right)$
Distribute $7k$ through the parentheses$7{k}^{3}-14{k}^{2}+49k-3\left( {k}^{2}-2k+7 \right)$
Distribute $-3$ through the parentheses$7{k}^{3}-14{k}^{2}+49k-3{k}^{2}+6k-21$
Collect like terms$7{k}^{3}-17{k}^{2}+49k+6k-21$
Collect like terms$7{k}^{3}-17{k}^{2}+55k-21$