High school physics problem... easy but confused.. can u help me please?

Question

You are designing a pendulum designed to wreck buildings called a "wrecking ball". When the ball is displaced from its equilibrium position by 30 degrees on a 20 meter line, you would like it to have energy equal to 35,000 Joules when it returns to its equilibrium position. What mass must the wrecking ball have to accomplish this?

Answer 1

As you know work equal force times the distance.
W=F S
As you displace the line by an angle of A the force that wants the ball back to its resting place F is
F=mg sin(A)
The distance S the ball will travel to its equilibrium position is
S=L sin(A) so

W=mg sin(A) L sin(A)
W=mg L sin ^2(A)

and finally
m=W/(g L sin ^2(A))