(d/2) + (f/4) = 6
4*[ (d/2) + (f/4) ] = 4*6 ... see note below
4*(d/2) + 4*(f/4) = 4*6
2d + f = 24
2d + f - 2d = 24 - 2d
f = 24 - 2d
The final answer is f = 24 - 2d
Note: I multiplied both sides by 4 because this is the LCD (lowest common denominator). Multiplying both sides by the LCD will clear out the fractions.