Question 668802
Parenthesis
Exponents
Multiplication/Division
Addition/Subtraction 

4*5+9-2*(1+2)  

Use parenthesis so that (1+2) = 3

4*5+9-2*3

Do multiplication:

20 + 9 - 6  since 4*5 =20 and 2*3 = 6

Finishing this up gives us 23.