You need to show us what the triangle is
We can use Pythagorean's Theorem in order to find the remaining side length
a^2+b^2=c^2
Let a be the shadow length
Let b be the height of the flagpole
Let c be the hypotenuse
28^2+b^2=35^2
784+b^2=1225
b^2=1225-784
b^2=441
b=√441
b=21
Therefore the flagpole is 21 meters tall.
Hope this helps!
The double angle formula for the sine is

The right hand side is exactly your expression, where 
So, rewriting the expression from right to left, we have

The LCD (least common denominator) is the lowest number that both denominators (12 and 5) go into. The lowest number that both 5 and 12 go into is 60. The LCD of the two fractions is 60.