Did you make a sketch?
From the first angle, you will get another angle of the triangle formed. It is 180 - 75 = 105 degrees. From there, you can find the last angle in the triangle. It gives 180 - (105+45) = 30 degrees.
(Or you use the property that the outside angle is equal to the sum of the two opposite angles in the triangle, and get 75 - 45 = 30 degrees)
Now, you can use the sine rule.
Solve for L, the distance from the first observer to the cloud, and you should get 848.5 m.
Now that you have that, draw a perpendicular line from the horizontal to the cloud. You get another triangle, this time a right angled triangle.
use the sine ratio to find the height of the cloud.