Answer:
[tex]3d+2f=21[/tex]
[tex]2f = 21 - 3d[/tex]
[tex]f = \frac{21 - 3d}{2} [/tex]
[tex]3d + 2f = 21 \\ 2f = 21 - 3d \\ f = \frac{21 - 3d}{2} [/tex]
