Let's call bicycles 'b' and unicycles 'u'
Note that bicycles have 2 tires, and unicycles have 1 tire.
We can make two equations:
b = u + 8
2b + 1u = 46
Solving the first equation for u, we get:
u = b - 8
Plug that equation into the second, and we get:
2b + (b - 8) = 46
Subtract 2b on both sides.
1(b - 8) = 46 - 2b
Basically, I used b and your question used n.
The correct answer is: D. 1(n - 8) = 46 - 2n.
