21=6+3u

Solution for 21=6+3u equation:

Simplifying
21 = 6 + 3u

Solving
21 = 6 + 3u

Solving for variable 'u'.

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

Add '-3u' to each side of the equation.
21 + -3u = 6 + 3u + -3u

Combine like terms: 3u + -3u = 0
21 + -3u = 6 + 0
21 + -3u = 6

Add '-21' to each side of the equation.
21 + -21 + -3u = 6 + -21

Combine like terms: 21 + -21 = 0
0 + -3u = 6 + -21
-3u = 6 + -21

Combine like terms: 6 + -21 = -15
-3u = -15

Divide each side by '-3'.
u = 5

Simplifying
u = 5