For these two, you'll want to use the equation a^2 + b^2 = c^2, a and b are the legs of the triangle (the shorter one going across and the other going up straight), and c is the hypotenuse (the diagonal leg that connects a and b).
So #1 for example, (6 ft)^2 + x^2 (the depth of the water) = (15 ft)^2. 36 ft +x^2 = 225 ft. When you solve for x, you should get approximately 13.75 ft.
Now for #2, you are given that a = 3 and c = 5, plugging those numbers into the equation will give you b :)