We will determine the area as follows:
So, the area of the sector is approximately 21.2 square units.
(The reason it is multiplied by 3/4 is that the normal area of a circumference is given by pir^2 [Whole circle], but since it is stated that the missing area has an angle of 90°, then the remainder of the circle [Area to find] is 3/4 of the original are).