A circle in its nature is having 360degrees as the sum of its interior angles. by merely observing the properties of the circle we can say that 90degrees is just 1/4 of the whole circle.
Area of the circle as we know is A= π r²
knowing that the unshaded region is 1/4 of it then we can say that that the shaded region is 3/4 of the area of the circle. then you'll have it as:
A=3/4 π r²
r is given as 28mm. substituting the given you'll have:
A=3/4 π (28mm)²
A = 588 π mm²
or in decimal
A = 1,847.256 mm²