x =-2 + tcos30° = - 2 + tsin60° 即：x = -2 + tsin60° (1)
y = 3 - tsin60° (2)
(1) + (2)得：x + y = 1
y = - x + 1
直线的斜率 k = -1,即为直线倾斜角的正切值
所以 tana = - 1,
所以 a = 135°