Since the inside figure is a square, the base of the triangle must be 5.
To find the height: The length of the entire figure is 17; subtract 5 to account for the square, and you know that the remaining 12 must be divided easily on the right and left side; this shows that the height of the triangle on the left must be 6.
Plug these numbers into the formula: 5 * 6 / 2 = 15 :)
