Given the formula a=b+c, rewrite the formula to solve for b.

a=b+c
This type of problem can be done in multiple ways.
Start by subtracting b from both sides 
a-b=b-b+c

Now we have: 
a-b=c

Subtract a from both sides: 
a-a-b=c-a

Now we have: 

-b=-a+c

Divide both sides by -1 (because we have -1b) 

Final  
b=a−c