There isn't sufficient information if the shape is a triangle, rectangle or parallelogram, but if the shape is a square ABCD then AB=BC. There seem to be typos in the lengths of AB and BC, so, assuming AB=3x+2 and BC=5x-10, 3x+2=5x-10, 12=2x, x=6, making AB=BC=20. Also, AC is a diagonal so:
AC2=AB2+BC2=400+400=800. Therefore AC=√800=20√2=28.28 approx.
Another possible answer is that A, B and C lie on a straight line so AC=AB+BC=3x+2+5x-10=8x-8.
If BC=5x+10, then AC=3x+2+5x+10=8x+12.
Please check the details of your question and see if any of the solutions meet the criteria of the proper question.