Gauge pressure equation for mercury height in two-fluid manometer

[Precursor]

Homework Statement

The attempt at a solution

To find the height of mercury I have used the following equation:

$$P _{gauge}=\rho _{air}\cdot g\cdot h_{air}+\rho _{Hg}\cdot g\cdot h_{Hg}$$

where $$P _{gauge}=3.75 KPa$$

So I solve for $$h_{Hg}$$ and get 2.8 cm. I don't think this is correct. Am I even using the correct equation?

 

[Precursor]

Anybody please?