The weight of the sign is 30 N. It is suspended from a ceiling by two cables that are 2 m long and which are attached to the ceiling by bolts that are 3 m apart.
The tension in each of the cables can be divided into a component parallel to the horizontal and a component perpendicular to the horizontal. The net effect of the component parallel to the horizontal is 0 for the two cables.
The component perpendicular to the horizontal adds up to 30 N. As the cables are 2 m long and are attached by bolts 3 m apart the angle they make with the horizontal is arc cos (1.5/2) = 41.40 degrees.
If the tension in the cable is T, T*sin 41.40 = 15
=> T = 22.67 N
The tension in the cables is 22.67 N.