Chapter 11 - 11.2 - Arithmetic Sequences and Partial Sums - 11.2 Exercises - Page 786: 15

See below.

Plugging in the different values: $a_1=3-4(1-2)=3-4(-1)=3+4=7$ $a_2=3-4(2-2)=3-4(0)=3+0=3$ $a_3=3-4(3-2)=3-4(1)=3-4=-1$ $a_4=3-4(4-2)=3-4(2)=3-8=-5$ $a_5=3-4(5-2)=3-4(3)=3-12=-9$ $a_5-a_4=a_4-a_3=a_3-a_2=a_2-a_1=-4$, thus this is an arithmetic sequence with a common difference of $4$