SInce you need 1.5 feet of overhang, add 3 feet to each axis dimension 1.5 on each side):
Minor Axis = 18 + 3 = 21 feet
Major axis = 25 + 3 = 28 feet
The area of an ellipse is found by multiplying half the minor axis by half the major axis by PI.
1/2 minor axis = 21 / 2 = 10.5
1/2 major axis = 28 / 2 = 14
Using 3.14 for PI
Area = 10.5 x 14 x 3.14 = 147 x 3.14 = 461.6 sq ft
The first digit of the password can be any of the ten digits - 10 possibilities.
A digit cannot be repeated, so the second digit can be any digit except the one you chose for the first digit - 9 possibilities.
The third digit can be any digit except the digits you chose for the first and second digit - 8 possibilities.
Using the rule of product:

There are 720 passwords.
3.82 x 10⁷ = 38,200,000 (move the decimal 7 places to the right)
So get b alone.
A=bh
Divide by h
A/h=b
b is alone
And just to rearrange:
b=A/h