To solve it for "a" is to isolate "a' in one side, by doing some algebraic operations.
U =ak -b
1) Let's rewrite it
-b+ak=u
2) Add b to both sides
-b +b +ak = u +b
ak = u+b
3) Divide both sides by k
[tex]\frac{ak}{k}=\frac{u+b}{k}[/tex]4) Finally, we have it for 'a':
[tex]a\text{ =}\frac{u}{k}\text{ + }\frac{b}{k}[/tex]