Hi there!
Since this triangle is a right triangle and a 45 degree angle is shown, we can assume that we'll need to use the 45-45-90 relationship. Using this relationship, we know that a is equal to the hypotenuse divided by the square root of 2. Since there cannot be a radical in the denominator, we need to multiply the fraction by root 2 over root 2 (or 1) and simplify.
WORK:
28/squareroot(2) x sr(2)/sr(2)
28 sr(2) / 2
a = 14 squareroot(2)
Hope this helps!! :)
If there's anything else that I can help you with, please let me know!