已知三元一次方程组$\left\{\begin{array}{c}x-y=1\\ x+z=2\\ z-y=17\end{array}\right.$，则x﹣y+z的值为　____ 

     What is color? Why do some of the things around us look red, some green, others blue?
    Colors are really made by deflected (反射) light. We see color because most of the things reflect light. In the same way, if something is green, it reflects most of the green light. If something reflects all light, it is white. If it doesn't reflect any light, it is black.
Some of the light is reflected and some is taken in (吸引) and turned into (变成) heat (热能).The darker the color is, the less light is reflected, the more light is taken in. So dark-colored clothes are warmer in the sun than light-colored clothes.