A horizontal line will always take the form y = b, where b is the y-intercept. Remember, the slope of a horizontal line is 0. Your y-intercept will be the y coordinate of your point.
Recognize that the slope is
The slope of a perpendicular will be the negative reciprocal of this slope. That is, we invert the fraction and change its sign. This slope becomes:
Now, this is where I'm having difficulty understanding which line you are trying to define.
What do you want to do? Are we defining a line perpendicular to 2x + 5y = 10 from (-2, 3) or are we defining a line perpendicular to 2x + 5y = 10 from (-1, 3)? Or is it something else entirely?