7. Given that the six interior angles of a regular hexagon are all $120^{\circ}$, and the lengths of four consecutive sides are $1, 9, 9, 5$ cm respectively. Then, the perimeter of this hexagon is $\qquad$ cm.
7. 42. (Hint: Extend the three pairs of opposite sides of the hexagon, the result is an equilateral triangle, it is easy to know that the side length of the equilateral triangle is 19 cm, thus find the other two side lengths of the hexagon to be 13 cm and 5 cm.)