A hydrostatic equilibrium question?

Question

A child attempts to drink water through a 115 cm long straw, but finds that the water rises only 50 cm. By how much has the child reduced the pressure in her mouth below atmospheric pressure?

Answer 1

assume water has a density of 1g/cm^3. call cross area of straw 'a'.
the weight of water is a x 50 x 1 x 9.8. the vacuum pressure (P) required to lift it is P x a so
P x a x (a conversion factor to account for Pressure in N/m^2 vs area in cm^2) = a x 50 x 1 x 9.8 x (a conversion factor to account for grams on one side and Newtons on the other)
the a's cancel.
can you get it from there?