write the equation of the passing through the point (6,-8) and perpendicular to the line y=(-5/8)x+25.
The slope of the line you want to be perpendicular to is (-5/8), so the slope of the line that you want is 8/5. That is because the product of the slopes of perpendcular lines must be -1.
So your line must be of the form
y = (8/5) x + C
Choose C so that it passes through the point (6, -8)
-8 = (8/5)*6 + C
C = (-40 -48)/5 = -88/5