Area of rectangle  =    Area of HMN + Area of HLO + Area of ONH

              9×6                        =    1/2 x 2 x 6       +  1/2 x 7 x 6    + 1/2 x (OH) x (NK)

               9×6                       =             6               +     7×3            +   1/2 x √(6²+7²) x (NK)            

               NK   =   (54 – 6 – 21) x 2 / √85  = 5.86cm (to 2 d.p. or 3 sig. fig)