Yes you are correct B is the right answer
Answer: 
Step-by-step explanation:
Having the following equation given in the exercise:

You can solve for "a" following this procedure:
1. You can apply the Subtraction property of equality and subtract
from both sides of the equation:

2. Now you must subtract the terms on the left side of the equation. Notice that the Least Common Denominator is "d". Then:

3. Finally, you can apply the Multiplication property of equality and multiply both sides of the equation by "b". So, you get:

A "system" of equations is a set or collection of equations that you deal with all together at once. Linear equations (ones that graph as straight lines) are simpler than non-linear equations, and the simplest linear system is one with two equations and two variables.