...the term "ac" ...no idea what is being referred to.  The simplification you provided is identifying common terms and add/subtracting in order to isolate the variable on the leftside and any numeric term on the rightside.  Hence, on the leftside the addition "4b2-16b-2b" results in "-10b" and the rightside reduces to "-20".  The final result in division of both sides by -10, leaving "b=2". 

It is possible that your inquiry is really meaning "4b^2" rather than "4b2".  and that results in a solution set {5/2, 2}.  If so, then perhaps "ac" is referring to "alternating current".