Hi there!
Since we know that this is a right isosceles triangle, we can use the 45-45-90 relationship to figure out the missing side lengths. Using this relationship, we know that the two marked sides are both equal to 5 inches. Next, we need to use the formula to find the area of a triangle, which is A = 1/2(b * h). We know that the base of this triangle is 5, as is the height. All we need to do is plug in the numbers and solve: A = 1/2(5 * 5) | A = 1/2(25) | A = 12.5 inches^2.
Hope this helps!! :)
If something doesn't make sense or if there's anything else that I can help you with, please let me know!