HIIII, lol. could you see what i did .. wrong please? :/
see the question is basically asking what is the time taken for the boat to travel the dist AC, dist BC=54 because the speed of the liner is 36 km/h and the time it travels is 1.5 hrs thus the dist=1.5*36=54, the dist AC can be found out by the cosine rule, x=angle ABC
A^2=B^2+C^2-2ABcosx
A^2=90^2+54^2-(2*54*90*cos 135)
A=AC=133.75 km
therefore the speed of the boat = 133.75/1.5=89.2 km/h