The slope for a line that's perpenficular is the resepical of each other
Ex. Slope: -2 Perpendicular Slope to 1st Line: \frac{1}{2}
so the perpendicular slope to 7/8 is -8/7
for y-intersecpt i would go on desmos(graping calculator website) and type in the equation y=-8/7x+b
Use the slider, and it you give you... You can't find the whole equation (y=mx+b). Because as long as the line has the slope of -8/7 it's perpenicular.
Did the problem tell you anything about a point that this new line goes through? Or no?