The tricky part is running 3/4 pace should make the horse go slower over the distance of 1100m
I see you have done a 68.75 divided by 4 multiplied by 3 to get 51.5 secs.
-nogojoe
Sorry! I got so caught up in making the quarter horse joke--where I was expecting the number to be lower/faster--that I didn't notice that I accidentally multiplied 68.75 seconds by 0.75 instead of dividing by it. 🤦♂️
68.75 / 0.75 gives about 91.6, which is the same answer ConstanceJill came to using other methods, which admittedly are easier to follow.

The correction in full (showing my work this time):
The horse takes 75 seconds to run 1200 meters. That is 0.0625 seconds per meter. (75 / 1200 = 0.0625)
At that pace, it will take 68.75 seconds to run 1100 meters. (0.0625 * 1100 = 68.75)
Running the 1100 meters at 0.75 pace would take about 91.6 seconds. (68.75 / 0.75 = ~91.66667)