The answer is B) B = -28.
Here are the steps I did.
1. Simplify 3/4B to 3B/4 ( -3B/4 = 21 )
2. Multiply both sides by 4 ( -3B = 21 * 4 )
3. Simplify 21 * 4 to 84 ( -3B = 84 )
4. Divide both sides by -3 ( B = -84/3 )
5. Simplify 84/3 to 28 ( B = -28 )