( 2x,\ x+8,\ 3x+1 ) are in A.P.

For three terms ( a, b, c ) in A.P.: ( 2b = a + c )

[ 2(x+8) = 2x + (3x+1) ] [ 2x + 16 = 5x + 1 ] [ 16 - 1 = 5x - 2x ] [ 15 = 3x ] [ x = 5 ]

Answer: ( x = 5 )