42=b-21

Solution for 42=b-21 equation:

Simplifying
42 = b + -21

Reorder the terms:
42 = -21 + b

Solving
42 = -21 + 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.
42 + -1b = -21 + b + -1b

Combine like terms: b + -1b = 0
42 + -1b = -21 + 0
42 + -1b = -21

Add '-42' to each side of the equation.
42 + -42 + -1b = -21 + -42

Combine like terms: 42 + -42 = 0
0 + -1b = -21 + -42
-1b = -21 + -42

Combine like terms: -21 + -42 = -63
-1b = -63

Divide each side by '-1'.
b = 63

Simplifying
b = 63