Question 469825
What this problem is asking you to do is to solve for a. 
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Do you recognize why the problem says that a is NOT equal to b? It's because if a = b then the denominator of (b - a) would be equal to zero, and as you know, division by zero is not permitted in algebra.
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So the problem you were given is:
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{{{(ab)/(b - a) = 1}}}
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First get rid of the denominator (b - a) by multiplying both sides of the equation by (b - a) to get:
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{{{ab = b - a}}}
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Next add +a to both sides to get rid of the -a on the right side.  When you do, the equation becomes:
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{{{ab + a = b}}}
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Factor the left side by removing a from both terms to get:
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{{{a(b+1) = b}}}
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Solve for a by dividing both sides by (b+1) and you have:
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{{{a = b/(b+1)}}}
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Notice that this is answer B in your list of possible answers.
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Hope this helps you to understand the problem.