It is impossible to know the solution of this expression unless you know how much a is worth or some information on the other side of the equation. If you don't know what I mean then I'll explain it a bit more. An equation is 2 expressions that are equal to each other but here I don't know anything about the other side of the equation. For example, an equation like this: 3a - 15= 6 is solvable because I know enough information to solve this.
3a - 15 + 15= 6 + 15
3a = 21
Therefore I can now find out what the answer is but your equation does not have enough information for it to be solved.
