Solve for c a = (b - c) / c

(b-c) / c can be separated into b/c-c/c, which is the same as b/c - 1.

so

a = b/c - 1

add one to both sides of equations

a+1 = b/c - 1+1

1 cancels out on the right side leaving

a+1 = b/c

multiply both sides by c

c * (a+1) = (b/c) * c

c cancels out on the right side making

c * (a+1) = b

divide both sides by a+1

c = b / (a+1)