To answer this question, you need to know the formula to count rectangular area and perimeter.
You already have given the length of rectangular(3/4mi) and the area(1/2 mi square). Then, using the area formula you can determine the width. The calculation would be:
Area= length * width
1/2mi square= 3/4 mi* width
width= (1/2)/(3/4)= 4/6= 2/3 mi
Then the perimeter should be:
perimeter= 2*(length + width)= 2* (3/4 mi+2/3mi)= 34/12mi= 2.83 mi