Solve for a in the equation: 6/a = 5/b +2/u
make the lhs a single fraction by using bu as a common denominator
6/a = 5u/(bu) + 2b/(bu)
6/a = (5u + 2b)/bu
Now that you have a fraction on both sides, simply invert them
a/6 = bu/(5u + 2b)
multiply both sides by 6 to get a by itself
a = 6bu/(5u + 2b)
Answer: a = 6bu/(5u + 2b)