var expr = new Expression("x");
expr = expr.subtract(3);
expr = expr.add("x");
console.log(expr.toString());
2x - 3
var eq = new Equation(expr, 4);
console.log(eq.toString());
2x - 3 = 4
var x = eq.solveFor("x");
console.log("x = " + x.toString());
x = 7/2