\[(3a - 3x)(7a - 7x) =\]
\[= 3 \cdot 7 \cdot (a - x)(a - x) =\]
\[= 21 \cdot (a - x)^{2} =\]
\[= 21 \cdot \left( a^{2} - 2ax + x^{2} \right) =\]
\[= 21a^{2} - 42ax + 21x^{2}\text{.\ }\]