What is the formula to convert square inches to linear yards ??

Here is the problem:

A crocheter has created an isosceles trapezoid (U.S.math definition) shawl on paper to convert to a wearable garment. The intended measurement of the finished product is 65" on the top, 48" on the base, 30 inches high. Formula used for the area of the trapezoid: A = 65 + 48/2 x 30 = 1695 sq. inches.

The test swatch measures 6 x 9 inches, which converts to 54 sq. inches. 50 yards of yarn was used in the test swatch.

How many yards of yarn will the crocheter have to purchase ?? Please provide the formula, as this is part of a syllabus.

Thanking you in advance.