There's a fair amount of arithmetic to do to get the final result.
You might save cycles by doing an initial check whether the bounding box around the ball intersects the bounding box around the line -- depending on the length of the line and its angle.
The other approach may be to avoid performing a calculation on every (dx, dy) step. In particular, if the ball is moving in a straight line (or any other path that you have a polynomial for), you can calculate where the centre will be if it meets the line, then after each step you can check if the centre has gone past that point (the bounding box (x, y, x+dx, y+dy) encloses the impact point). Until, of course, the ball changes direction.
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