Answer to the third problem:
We know that the area of a triangle is 1/2 * bh
Area = bh/2
Area = 20
h = h
b = h+6
20 = h*(h+6)/2
20 = (h^2 + 6h)/ 2
$\displaystyle 20 = \frac{1}{2}h^2 + 3h$
$\displaystyle 0 = \frac{1}{2}h^2 + 3h - 20$
$\displaystyle h = ...$
Use the same method to solve the other one that is similar.
If you know how to solve the third one, why not the first one? They are exactly the same. Unless you did trial and error to get the answer.
For the second one, use the pythagorus theorum:
$\displaystyle a^2 = b^2 + c^2$
One leg is 2000ft long, b = 2000
The hypotenuse is 400ft longer than the other leg, a-400 = c
We substitute these known values into the equation and get:
$\displaystyle a^2 = 2000^2 + (a-400)^2$
$\displaystyle a^2 = 2000^2 + a^2 - 800a + 160000$
Now simplify and use the quadratic formula to solve for a, which is the length of the hypotenuse. (You might get 2 answers. In that case, use the one most appropriate)