I hope this helps you

Add 3b to both sides  [latex]ac=c-4+3b [/latex] Subtract c from both sides  [latex]ac-c=-4+3b [/latex] Regroup terms [latex]ac-c=3b-4 [/latex] Factor out the common term c [latex]c(a-1)=3b-4 [/latex] Divide both sides by a-1 [latex]c= dfrac{3b-4}{a-1} [/latex]