H -7/J = k solve for J

Manuela solved the equation below. what is the solution to manuela’s equation?

Abag contains 5 red marbles , 6 blue marbles and 4 green marbles. susie selects a marble and returns it to the bag before selecting another marble. if susie selects a red marble 6 out of 20 times, what is the experimental probability that the next marble she selects will be a red marble ?

Hello i need with my math homework i don't understand can someone explain how this works