Solution - Simplification or other simple results

Step  1  :

Equation at the end of step  1  :

  (0 -  7a2) +  21a

Step  2  :

Step  3  :

Pulling out like terms :

 3.1     Pull out like factors :

   21a - 7a²  =   -7a • (a - 3) 

Final result :

  -7a • (a - 3)