SOLUTION: The 1st,2nd and 3rd term of an arithmetic progression are 8-x,3x and 4x+1..find x

, 

  (8-x)+d = 3x, 3x+d = 4x+1
    8-x+d = 3x, -x+d = 1
    -4x+d = -8     d = 1+x

     -4x+d = -8 
 -4x+(1+x) = -8  
   -4x+1+x = -8
     -3x+1 = -8
       -3x = -9
         x = 3

d = 1+x = 1+(3) = 4

Checking:  
1st term = 8-x = 8-3 = 5
2nd term = 3x = 3(3) = 9
3rd term = 4x+1 = 4(3)+1 = 12+1 = 13

5,9,13 form an arithmetic sequence.

Edwin