add d to both sides
c(a+b) = f + d
divide away c
a+b = (f+d)/c
subtract b from both sides:
a=[(f+d)/c]-b
c(a + b) - d = f
c(a + b) = f + d
[tex] \frac{c(a+b)}{c} = \frac{f+d}{c} [/tex]
a + b = [tex] \frac{f+d}{c} [/tex]
a = [tex] \frac{f+d}{c} - b [/tex]
Answer:
a = -b + [tex] \frac{d+f}{c} [/tex]