For this I-Beam which has a depth of 50mm I need to determine the Principal and maximum shear stresses acting on an element mid way between the Neutral Axis and the top of the flange (12.5mm from the top of the flange).

I don't know how to determine sigma_x and sigma_y and tau_xy.

I know Stress=Force/Area

so does this mean for an element of unit area 1mm^2 i use

Shear Stress = Shear Force/1 = 25/1=25 Pascals?

Is this on the right track or am i entirely using the wrong values and formula?