y - -10 = - (x - 3)
y + 10 = -x + 3
y = -x - 3
When put in point slope form, the equation is written as y minus negative 10 equals x minus 3 times -1 (y - - 10 = - (x - 3). The y minus negative 10 changes to y plus 10 (y + 10) and you distribute the right side by multiplying by negative one (-1 or simply just "-"). Then you subtract 10 from both sides, leaving us with the answer y = -x - 3.
Hope that made sense and helped.