How do you check 1 - x = 16 - 4x when the x is 5?

Simply

1 - x = 16 - 4x when the x is 5:

Substitute x for 5, by replacing x with 5.

1 - (5) = 16 - 4 (5)

1 - 5 = 16 - (4 * 5)

1 - 5 = 16 - 20

-4 = - 4

LHS - RHS

It is better to show this by doing each side individually - Left Hand Side (LHS) and Right Hand Side (RHS).

LHS

1 - x when x = 5

1 - 5 = - 4

RHS

16 - 4x when x = 5

16 - 4 (5) = 16 - 20 = - 4