What is the soultion of - 21 = n - 8
what is n

This is a simple equation:)
We have to isolate "n" to be alone. 
-21 = n - 8 

First add 8 to both sides. 
-21 = n - 8
+8  =    + 8 it cancels out on the right side of the equal side and you solve on the left side

s
-21 + 8 = -13
-13 = n 

Finally our answer is
-13 = n  

When in doubt, eat a pineapple:)