21=6+b

Solution for 21=6+b equation:

Simplifying
21 = 6 + b

Solving
21 = 6 + b

Solving for variable 'b'.

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

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

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

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

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

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

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

Simplifying
b = 15