Answer:
x = -3
Explanation:
The line y = 3 is a horizontal line. So a perpendicular line is a vertical line. The form of the equation for a vertical line is 
"x = anumber" 
The information we have to work with is the point (-3, 10) A point is in the form (x,y). So we know that x is -3 and y is 10. 
We need the x. 
x = -3 
This is the equation of the vertical line through (-3,10). It is perpendicular to the horizontal line
 y = 3.
Answer:
x = -3