The first solution 2
Draw a line perpendicular to the river after point A (this line is called capital A), and the intersection of the two lines is the dock location.
Solve again 1
Draw a straight line perpendicular to A through point A (this line is called capital B), and B is the road parallel to the river.
Re-solution 3
This question mainly depends on you, because there is no plan and no unit.
A is the radius. Draw this circle with a compass, then calculate the actual distance (that is, the actual length of A) to get C(C is the actual length of A), and multiply the square of C by pi to get the area.