A rectangle has length L and width L - 3. If P(L) represents the perimeter as a function of L, what is P(10)?

The perimeter of a rectangle is twice the sum of its length and width: $P(L)=2\big(L+(L-3)\big)=2(2L-3)=4L-6$. At $L=10$: $P(10)=4(10)-6=34$, choice C. Check directly: the length is $10$ and the width is $7$, so the perimeter is $2(10+7)=34$. Choice A ($17$) is the sum of the length and the width, which is only half the perimeter. Choice D ($40$) is $4L$, forgetting to subtract $6$. Choice B ($26$) uses a width of $L+3=13$ instead of $L-3=7$.