Draw a backwards L. Connect the two lines with a third line. You should now have a right-angled triangle.
The SPEED of the rifle is the diagonal line. It is 300.
The DISTANCE to the target is the horizontal line. It is 2500.
Imagine the gun is on the bottom-left corner of the triangle. The angle of elevation is this corner of the triangle. Angles of elevation are always measured UP from the horizontal.
Using simple trigonometry:
Edited: clearer explanation of the triangle