Question

how to find area of circle if diameter is given

Answer 1

generally in questions they will give the radius but here they have given the diameter which is twice the radius.
for example if the they have given diameter as 4 cm 
area of circle =pi r^2
here diameter is given so,
radius will be half of diameter 
area= pi d/2^2(d=diameter)

Answer 2

area= π x(d/2)^2 (in m^2)
where,
d - diameter of circle (in m)