20 - 2 * 3^2
Remember PEMDAS.
Parentheses
Exponents
Multiplication/Division
Addition/Subtraction
There are no parentheses, but there is an exponent. So first you must do 3^2.
20 - 2 * (3*3)
20 - 2 * 9
Next is multiplication/division. Multiply -2 by 9.
20 - 2 * 9
20 - 18
Lastly, you subtract.
20 - 18
2
The answer is 2.