The middle is a rectangle, so we know the middle section is 10.
Each side is a triangle with a height of 12 and a hypotenuse of 15.
Using the Pythagorean theorem we can solve for the missing side.
leg = √(15^2 - 12^2)
leg = √225 - 144
leg = √81
leg = 9
There is a triangle on each side so X = 9 + 9 + 10 = 28