You have to remember PEMDAS
Please excuse my dear aunt sally,
Parenthesis exponents multiplication division addition subtraction.
Thats is the order that you work the problem.
Okay so first the parenthesis 21-18=3
There are no exponents so skip that
Now your equation should look like this ---> 3*11-5
Next is multiplication. Mulitply 3*11 which equals 33
There is no divison or addition so skip those steps
Now subtraction. By now your equation should look like this
---> 33-5
Just subtract 5 from 33 and you should get 28
So, (21-18)x11-5 =28
