Replace the loading by an equivalent resultant force and couple moment acting at point O $.$

Step I. Divide the load area into one rectangular and two triangular areas

Step $2.$ Find the equivalent force for each area in kN for rectangle FI and two triangles F$2$ and F$3$

Step $3.$ Find centroid for each area and write horizontal distance from O

Step $4.$ Draw each resultant in its centroid

Step $5.$ Find total resultant force FR$=$FI$+$F$2+$F$3$

Step $6.$ Find Resultant couple as sum of moments moment of each Force about O