The height of the sail is ##3m##.
The area of a triangle can be calculated as:
where ##b## is the length of base and ##h## is the length of height
From the text we see that the area is given ##A=3.75## the base is given: ##b=2.5##
Our task is to calculate the height ##h##
##A=1/2xxbxxh##
##2A=bxxh##
##h=(2A)/b##
##h=7.5/2.5##
##h=3##
The answer is :