Cloud base is 5600
(OAT-Dpt) x 400
(21-7) x 400
14 x 400 = 5600
Why?
Air cools at the Dry Adiabatic Lapse Rate until saturated at it Due Piont.
OAT is 21
-3degC per 1000 '
Reaches DP of 7 deg at 5600' and condensed out.
It then cools at the Saturated LR of 1.5 / 1000' and the tops are another story altogether, subject to inversions etc...