Is the turret also moving?
How good is was your maths teacher? The rigorous way is to solve for the intersection vector through a series of linear equations using a iterative implementation of Gaussian elimination. Khan Academy covers it well but this is not a beginner's approach.
Otherwise, you can always just simulate. Every ten frames or so you could (asynchronously on a thread if you can) loop the movement and test if they get close enough to collide.
Otherwise, I can't think of an easy beginner way, sorry.
Cast a raycast pointing in the forward velocity vector of the ball and and it'll tell you if it intersects with the turret.