So because of the way 30-60-90 triangles work, the side opposite to the 30 degrees is half of the side opposite to the 90 degrees. (If u finish the triangle, you’ll see the 30 degrees). So because of this fact, u know that the side opposite to the 90 degrees will be twice as much as 7 yards. This means her height will be 42 feet, which is higher than the tree.
You can also do
sin30 = 21/x
1/2 = 21/x
x = 42
42>40
