What is the vapor pressure of water at 750c?

There's not really enough information to answer your question.
I assume you're asking what the pressure of water vapor would be at 750C, which would require the use of the universal gas law.

PV = nRT, where P is the pressure of the gas, V is the volume, n is the number of mols of gas you have, R is the universal gas constant, and T is the temperature. 

It can be rearranged to be P =(nRT)/V
And you could find each variable independently to find the pressure. 

R is 8.314, as it always is (being a universal constant)
n can be found using n=m/18.02, where m is the mass of gas you're measureing
V is the volume of whatever container you're using, if it's a geometric shape you can just measure it
T is 750 + 273.15 = 1023.15, because temperature is always in kelvin and it was given in celecius in the question.

Ithink it is quantity of plants