As we can tell, the first number is 3, so we'll have a y-intercept in our equation and overall. Secondly, we can tell it's increasing by 7 each time, so we can tell that's the rate. But, it won't just be 3+7x, if we plug 1 in, it would be 10 and not 3. We need to put in the parenthesis 3+7(x-1). If we put one in, we would receive three. Let's make sure this works.
3+7(5-1)
3+7(4)
3+28
31
31 is the fifth terms.
So the expression would be 3+7(x-1).
Let's find the 100th term.
3+7(100-1)
3+7(99)
3+693
696
So the 100th term would be 696.