Parallel lines are lines that never cross each other, so they have the same slope. The slope of y= -10x is -10, so the line parallel to that also has a slope of -10.
Remember, slope-intercept form of a line is y = mx + b, where m is the slope and b is the y-intercept. y= -10x is in that format, where m = -10 and b isn't there, so b=0.
I hope that helps, and have fun graphing!
Liz Z.