Hello! :)
Answer:

Explanation:
PEMDAS
Parenthesis
Exponent
Multiply
Divide
Add
Subtract
Left/Right
First, you do parenthesis.
2+3-4+(5*4)
5*4=20
2+3-4+20
Then you add/subtract from left/right.
2+3=5
5-4+20
5-4=1
1+20=21

Hope this helps!
Thanks!
Have a nice day! :)
:D
-Charlie