Answer:
5.4
Explanation:
I'm assuming these figures are similar, so we can set up equal proportions to solve for x.
See how we have put all the values of Figure A's sides on the tops of the fractions, and Figure B's on the bottom. It won't work if one is flipped.
Cross-multiply.
7.2 * 3 = 4x
21.6 = 4x
21.6/4 = x
x = 5.4