b+2(-3-b)=21

Solution for b+2(-3-b)=21 equation:

Simplifying
b + 2(-3 + -1b) = 21
b + (-3 * 2 + -1b * 2) = 21
b + (-6 + -2b) = 21

Reorder the terms:
-6 + b + -2b = 21

Combine like terms: b + -2b = -1b
-6 + -1b = 21

Solving
-6 + -1b = 21

Solving for variable 'b'.

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

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

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

Combine like terms: 21 + 6 = 27
-1b = 27

Divide each side by '-1'.
b = -27

Simplifying
b = -27