.
When n is divided by 4, the remainder is 3 (given).
Hence, when 3n is divided by 4, the remainder is 3*3 = 9, which is EQUIVALENT to the remainder 1 modulus 4.
It implies, that when (3n-1) is divided by 4, the remainder is 0. ANSWER
Example. Let n = 7, giving the remainder 3 when divided by 4.
3n = 21, giving the remainder 1 when divided by 4.
(3n-1) = 20, giving the remainder 0 when divided by 4.