If the angle of depression is 28° the complementary angle in the triangle is 62°. With this you can use soh-cah-toa to find the x value.
cos62 = 350/x; multiply both sides by x
cos62 x = 350; divide both sides by cos62
x = 350 / cos62
x = 745.519
x = 746 ft
Hope this helps you!