The area of a triangle is 1/2 base height. If I call the 750 foot side the base then, since the angle between this side and the 500 foot side is a right angle, the height of the triangle is 500 feet, Thus the area is
1/2 base height = 1/2 750 500 = 187500 square feet.
Is the answer given in acres? There are 43560 square feet in an acre so the area of the lot is
187500/43560 = 0.43 acres