Physics
posted by Anonymous .
A bird is resting on a phone wire between two poles that are 30 m apart. The bird is exactly midway between the two poles. Because of the weight of the bird, the wire sags by 50 cm. The tension in the wire is 70 N

So what is the question? All you haven't been given is the weight of the bird.
Figure the angle at the center (vertical to wire) tanTheta=15/.5
Now, if tension is 70, and each side supports half the weight of the bird
then cosTheta=weight/2 / tension
and you solve for weight.