The x in the formula is the depth.
You are given two different depths, replace x with the depths, solve for T, then subtract the to find the difference:
At 1200 meters the temperature is:
T = 4500/1200 = 3.75 degrees
At 3750 meters the temperature is:
T = 4500 / 3750 = 1.20 degrees.
The difference in temperature between the two depths is:
3.75 - 1.20 = 2.55 degrees C.