If you dont follow the right steps in the right order or properly, you won't end up with the right answer:
For example 3 x 5 + 7
If you do multiplication then add you get 22 which is correct.... but!!
If you add then do multiplication you get 36 which isn't right
Remember always follow PEMDAS
P-Parenthesis
E-Exponents
M or D- Multiplication or division (go left to right)
A or S- Addition or Subtraction (go left to right)