21=2f+f

Solution for 21=2f+f equation:

Simplifying
21 = 2f + f

Combine like terms: 2f + f = 3f
21 = 3f

Solving
21 = 3f

Solving for variable 'f'.

Move all terms containing f to the left, all other terms to the right.

Add '-3f' to each side of the equation.
21 + -3f = 3f + -3f

Combine like terms: 3f + -3f = 0
21 + -3f = 0

Add '-21' to each side of the equation.
21 + -21 + -3f = 0 + -21

Combine like terms: 21 + -21 = 0
0 + -3f = 0 + -21
-3f = 0 + -21

Combine like terms: 0 + -21 = -21
-3f = -21

Divide each side by '-3'.
f = 7

Simplifying
f = 7