400 - 3x = 163
Add 3x to both sides.
400 = 163 + 3x
Subtract 163 to both sides.
237 = 3x
237/3 = 79

is already in simplest form.
But if you meant to say

, we would combine the first two terms.
Adding/subtracting like terms is the same as adding/subtracting whole numbers.

Therefore:

Which gives us:
Answer:
Step-by-step explanation:
B