Question 438685
1. Common difference is 3. Assuming we start with the first term denoted {{{a[1] = 15}}}, we can write {{{a[n] = 15 - 3(n-1)}}} (after some practice you'll be able to instantly find an expression for the nth term, then simply change n to n-1 or n+1 to satisfy the sequence).


2. If they are in arithmetic progression, then


{{{a[2] - a[1] = a[3] - a[2]}}}


{{{(2x+2) - (3x-1) = (2x-1) - (2x+2)}}}


{{{-x + 3 = -3}}}


{{{x = 6}}}.