Yes, this is possible using Lua's loadstring function, although it would require a bit more work with function environments and such...
What exactly do you want to achieve with this?
Welcome to the forums. There is no feature in lua to evaluate expressions with that notation. If you really want to use this string as is, you would have to parse it. If you're allowed to change the notation, you could write it as standard lua, eg. `"5*a+10"`, and use `loadstring` to evaluate it into a function. `setfenv` would then be able to insert `a` into the function environment and you would be able to calculate the result.