Answer:
The common difference is 4.
Step-by-step explanation:
[tex]a_n=a_1+(n-1)d[/tex] You get to term number n by starting with term number 1 and adding the common difference d n-1 times.
[tex]a_1=3\text{, and }a_8=31[/tex]
[tex]a_8=a_1+(8-1)d\\\\31=3+7d\\\\28=7d\\\\d=4[/tex]
