18. Find the value of x.

Let's consider angles in the figure. The angle adjacent to $$70^{\circ}$$ is $$180^{\circ}-70^{\circ} = 110^{\circ}$$. The angle adjacent to $$80^{\circ}$$ is $$180^{\circ}-80^{\circ} = 100^{\circ}$$. The sum of angles in quadrilateral is 360, so: $$110+100+100+x = 360$$ $$310+x = 360$$ $$x = 360 - 310$$ $$x = 50$$ **Answer: 50°**