We can solve this as follows :
Let the height of the ceiling = H
So, from the observer's position........we have
tan (45) = x/H and tan (20) = (6 -x)/H
Where x and 6-x are the horizontal distances from the observer to lines drawn from each light perpendicular to the floor [ i.e, the ceiling height under both lights]
Solving for H, we have
x / tan(45) = (6-x)/tan(20) cross-mutiply
xtan(20 )= (6-x)tan(45) simplify
xtan(20) = 6tan(45) - xtan(45)
xtan(20) + xtan(45) = 6tan(45)
x[ tan(20) + tan(45)] = 6tan(45)
x = 6tan(45) / [ tan(20) + tan(45)] = about 4..399 m
So....the height of the ceiling is given by 4..399/tan(45) = about 4.399 m
P.S. ....check that (6 - 4.399)/ tan(20) also gives you, roughly, 4.399m
Here's a pic .....

The observer is at A and the lights are at D and C .... AB is the ceiling height = about 4.399 ... [4.4 rounded]
