The formula for the area of a parallelogram is the same as that for the area of a rectangle: A = bh, where b and h are the base length and height respectively. If the area of the parallelogram in question is 276 cm^2 and we divide this by 23 cm (which is half of the sum of its bases, or half of 46 cm), we get h = 12 cm.
The height of the parallelogram is 12 cm.
Note that dropping a height (line) from either of the upper two corners of the parallelograph forms a triangle with hypotenuse 13 cm, height 12 cm and base 5 cm.
