Answer :

Answer: 9r2+21r+9=-r2

Answer: c=110.25

Step-by-step explanation:

[tex]r^2+21r+c\\r^2+(2)(r)(10.5)+(10.5)^2=\\(r+10,5)^2\\Hence,\\c=(10.5)^2\\c=(10.5)(10.5)\\c=110.25[/tex]