Let's call the table height t. Then:
d + 80 = t + c
t + d = c + 100
These are 2 equations with 3 unknowns, so we can't solve all of it, but we're only interested in t.
Lets isolate d and put it in the second:
d = t+c-80
t + t+c-80 = c+100 => 2t = c+100 - c + 80 = 180 => t=90
So the table is 90 cm! You cannot determine Clement and Dmitri's height; only their height difference (Dmitri is 10 cm taller).