First get like things on the same side:
-5/6b - 1/2b= -2/3 - 3/4
Then find equivalent fractions to make the equation easier to solve.
-5/6b - 3/6b= -8/12 - 9/12
Then just add things together and solve.
-8/6b= -17/12
-8b= 6(-17/12)
-8b= -17/2
b= (-17/2)/-8
b= 17/16
You can then plug the answer back into the original question to make sure it is the correct answer.