So you know that for arithmetic sequences, the difference between each term is the same. So we can use this to form an equation:
Difference between term 2 and 1 = Difference between term 3 and 2
[tex](0.5x+7)-x = (3x-1)-(0.5x+7)
\\-0.5x+7=2.5x-8
\\15=3x
\\x=5[/tex]
So the three values are:
5, 0.5(5)+7, 3(5)-1
= 5, 9.5, 14