Help with physics Question?

Question

A sailboat race course consists of four legs, defined by the displacement vectors A,B ,C, D and , as the drawing indicates. The magnitudes of the first three vectors are A = 3.10 km, B = 5.50 km, and C = 5.10 km. The finish line of the course coincides with the starting line. Using the data in the drawing, find the distance of the fourth leg and the angle thetha

http://edugen.wiley.com/edugen/courses/crs1507/art/qb/qu/c01/ch01p_47.gif.

Answer 1

The site is not accessible without a password.
Can you supply angle information?

In general
The sum of x and you components must be zero.

Ax = Acos(a)
Ay =Acos(a) and so on

Ax+Bx+Cx+Dx=0
Ay+By+Cy+Dy=0

Solve for Dx and Dy
angle = arcTan(Dy/Dx)
and D=sqrt(Dx^2 +Dy^2)

That should do it!