-21=-3b-4b

Solution for -21=-3b-4b equation:

Simplifying
-21 = -3b + -4b

Combine like terms: -3b + -4b = -7b
-21 = -7b

Solving
-21 = -7b

Solving for variable 'b'.

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

Add '7b' to each side of the equation.
-21 + 7b = -7b + 7b

Combine like terms: -7b + 7b = 0
-21 + 7b = 0

Add '21' to each side of the equation.
-21 + 21 + 7b = 0 + 21

Combine like terms: -21 + 21 = 0
0 + 7b = 0 + 21
7b = 0 + 21

Combine like terms: 0 + 21 = 21
7b = 21

Divide each side by '7'.
b = 3

Simplifying
b = 3