If x is a positive integer and 4^x - 3 = y, which of the following CANNOT be a value of y?

Question: If x is a positive integer and 4^x - 3 = y, which of the following CANNOT be a value of y?

Solution:
If x = 1:
4¹ - 3 = 1

If x = 2:
4² - 3 = 13

If x = 3:
4³ - 3 = 61

The only value that y cannot take is 7.