The best choice would probably be the third option from the top. Since by using the distance formula you can solve for the length or distance of each of the sides of the triangle, and check to see if it is equal. As would be in an equilateral triangle.
Slope wouldn’t be best for the last leg or bottom leg as there is no change in y and x.
EF = 50 in.
In similar figures, corresponding sides are proportional. We use the similarity statement ABCD~EFGH to write a proportion:

Cross multiply the proportion:
45*x = 75*30
45x = 2250
Divide both sides by 45:
45x/45 = 2250/45
x = 50
The teacher would have to grade 322 problems
14 (problems) x 23 (students)
Answer: 322