Question 922689
First term (a)= -12 
15 th term  = 135
formula for  n th term   =a+(n-1)*d
 where n = no  of term
       d= common difference
    but 15 th term   = 135
            so 135  = a+(n-1)*d
               135 = -12+(15-1)*d
               135 = -12+14*d
             add 12 on both sides
            135+12 = -12+14*d +12
              147=14d
               divide with 14  on both sides
                  {{{ (147/14) = (14d/14)}}}
 10.5=d
common difference (d)= 10.5