The rules of which calculation comes first in an expression

They are:

. do everything inside parentheses first: ()

. then do exponents, like x²

. then do multiplies and divides from left to right

. lastly do the adds and subtracts from left to right

Example: 5 × (3 + 4) - 2 × 8 = 5 × 7 - 2 × 8 = 35 - 16 = 19