The centroid position along the x-axis can be obtained by integrating the function * x to get the moment about the y-axis, then divide by the area of the graph, all between x=0 to x=3.5m.
Expressed mathematically, x_bar=(∫f(x)*x dx )/(∫ f(x) dx limits are between x=0 and x=3.5m =15.278 m^3 / 6.1113 m^2 =2.500 m