Does anyone mind explaining this to me ?

I don't know about the circle, but from the way it looks, you can use pythagorean theorem (A^2+B^2=C^2). angle E would be the Right Angle making 65=C or in other words, the hypotenuse, with 60 and Line ED being the legs (it doesn't matter  which is A and which is B) leaving the equation

60^2+B^2=65^2 and solve from there. (if you need help solving, leave a comment and i will walk you through it in the comments)