Can i fix back pain by doing ab exercises?

Not always: If it were that simple we would have eliminated back pain in fit patients which is not the case. Core exercises are usually beneficial but if pain persists then other structures (facet joints or discs) may be damaged and in need of treatment.

Depends: If your back pain is due to poor core muscle strength, abdominal exercises can help depending on the type but should be combined with a flexibility program and cardiovascular one as well. The abdominal muscles are only a part of the core musculature needed for back stabilization.