A=c+(-10) solve for c in terms of other variables.

To do this we need to move 10 to other side.  To accomplish this you just need to add 10 to both side since (-10) 

so 
A+ 10 = c -10 + 10 
we get
A+ 10 = c

lets say it wasn't -10 but positive 10. 

A = c + 10  then we would subtract 10 from both sides

A -10 = c + 10 - 10 
we get 
A  - 10 = C 

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