You can use eval to convert a string into code.

function Start () {
    var a = Random.Range(1, 10);
    var b = Random.Range(1, 10);
    var c = Random.Range(1, 10);
    var operations = "-+*";
    var equation = "" + a + operations[Random.Range(0, operations.Length)]
        + b + operations[Random.Range(0, operations.Length)] + c;
    var x = eval(equation);
    Debug.Log (equation + "=" + x);
}

Yeah you will have to parse it, build up a tree to apply the calculation order etc.

I would say, if you randomly generate it, do not generate it as string in the first pace, but in your own format you can calculate and just present it as string.

Maybe have custom a function RandomEq(...) and get back the result and the string representation in a struct.

Completely random might be difficult/impossible, but you could easily build a function that churns out many different terms...

First I would use a Switch statement (and a random number) to choose the general structure of the term, without fixed numbers, only variables. Then you could assign the variables random numbers, calculate the solution and test it against the player's input.