The equation reads
----A·B
r=-------, solve for a
----A+B
Assuing that you're actually supposed to solve for A, and that the equation is meant to be as follows:
. . . . .\(\displaystyle r\, =\, \dfrac{A\, \cdot\, B}{A\, +\, B}\)
...the customary first step would be to multiply through, to clear the denominator. This will leave you with:
. . . . .\(\displaystyle r\, (A\, +\, B)\, =\, A\, \cdot\, B\)
Then multiply out the left-hand side, and then gather the terms containing the target variable on one side of the equation. (I'd gather then on the right-hand side.) Then factor out the target variable, and divide off whatever is left. Your answer should be in the form of a fraction.
If you get stuck, please reply with a clear listing of your thoughts and efforts so far. Thank you!