PEMDAS stands for the order to solve operations.
P is for parenthesis (or brackets), solve this first.
E is for exponents, solve this next.
MD is for multiplying and dividing.
AS is for adding and subtracting, this is done last.
a:
7 + 3 * 5 - 4 / 2 + 3
7 + 15 - 2 + 3
22 + 1
23
b:
[(7+3) * 5 - 4] / 2 + 3
[10 * 5 - 4] / 2 + 3
[50 - 4] / 2 + 3
46 / 2 + 3
23 + 3
26
I hope this helps you!