Explanation:
a general line equation is
y = ax + b
the original line is
y = 3/5 x + 10
the factor of x is always the slope (inclination of the line).
it is "y change/x change".
the slope of a perpendicular line (angle of 90°) is turning the slope of the original line upside-down and flips the sign.
so, it is -5/3.
we use the point information to get the right "b" for our new line :
-5 = -5/3 × 15 + b
-5 = -5×5 + b
-5 = -25 + b
b = 20
so, the perpendicular line equation going through (15, -5) is
y = -5/3 x + 20