INSTRUCTIONS:
1 . Expression contains numbers, mathematical operations, powers( ^ ), parenthesis and variables.
2 . If you use variables, you must input theirs numerical values.
3 . You can skip the multiplication sign. Example: 5*x^2 is the same as 5x^2.
4 . You can skip parentheses if they are not necessary. Example: $4 + (2\cdot3)$ is the same as $4 + 2\cdot3$.
5 . The examples of valid expressions are $1-3\,\left(2+4\right)$ and $\frac{a^2 + b^2}{a-b} + 4 \left(3-\frac{3}{4}\right).$