===============================
Rule: All four sides of a rhombus are the same length
So AB = BC
Use this to solve for x
AB = BC
3x+2 = x+12
3x-x = 12-2
2x = 10
x = 5
If x = 5, then
AB = 3*x+2 = 3*5+2 = 15+2 = 17
and
BC = x+12 = 5+12 = 17
Both sides are 17 as expected, so the proper x value is confirmed.
So DC is also 17 since AB = DC
