21=f+6

Solution for 21=f+6 equation:

Simplifying
21 = f + 6

Reorder the terms:
21 = 6 + f

Solving
21 = 6 + f

Solving for variable 'f'.

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

Add '-1f' to each side of the equation.
21 + -1f = 6 + f + -1f

Combine like terms: f + -1f = 0
21 + -1f = 6 + 0
21 + -1f = 6

Add '-21' to each side of the equation.
21 + -21 + -1f = 6 + -21

Combine like terms: 21 + -21 = 0
0 + -1f = 6 + -21
-1f = 6 + -21

Combine like terms: 6 + -21 = -15
-1f = -15

Divide each side by '-1'.
f = 15

Simplifying
f = 15