Hi Peyton M.
Thanks for the link to the picture
Since you have an Isosceles Right Triangle or a 45 45 90 Triangle you can simply apply the Pythagorean Theorem to calculate the hypotenuse or longest side
a2 + b2 = c2
In an Isosceles Triangle, a = b
In the picture at your link that is x; so a = b = x
x2 + x2 = c2
Can you continue from here to solve for c the diameter of the Circle?
Since your Isosceles Right Triangle is inscribed in the circle, the area of the shaded region would be
f(x) = Area of the Circle - Area of the Inscribed Triangle
I hope this helps please send me a message if you have any questions