The second choice! I hope my math is right!
The lenght of your ruler is 6inches or approximately 15cm.
I did the work for you but the answer would be 1 second
When the triangle RST is rotated by <span>90° in a clockwise direction, the name of the image after the rotation would be SRT.
The reason you get a name like this is because you're not only rotating the triangle, you also rotating the letters along with the image.</span>

This result is actually true for any exterior angle. The exterior angle of a triangle is equal to the sum of the two remote angles, and above is a short proof of it.