What is the gasoline-air surface tension its height in the capillary tube is 0.4mm and the density of gasoline is 700 kg/m^3?
The rise of the gasoline in a capillary tube is given by the relation:
h = (2*Sf*cos T)/(rho*g*r), where h is the height of the gasoline column, Sf is the gasoline-air surface tension, T is the contact angle, rho is the density of gasoline, g is the acceleration due to gravity and r is the radius of the capillary tube.
You have only provided the height of the liquid column and the density of the gasoline. It is not possible to proceed further without knowing the radius of the capillary tube and the contact angle as the height depends on it. For a broad capillary tube the height would be negligible whereas it would be substantial if the capillary tube is made narrow.
All I can provide from the information provided is that the value of Sf that you require is given by Sf = h*rho*g*r/ 2*cos T.
Here g is 9.8 m/s^2 and you know rho and h, substitute r and T and you can determine Sf.