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put this solution on YOUR website! If you draw a diagram you'll see that the length of XY plus the length of YZ = the length of XZ which is 28. You are given the lengths of XY and YZ, so make an equation.
12+2x = 28 Subtract 12 from both sides
2x = 16 Divide both sides by 2
x = 8
If x=8 then YZ = 2x = 2*8 = 16