The shortest distance between a point and a line is along a line perpendicular to the line. So, find the slope of the line perpendicular to y=-4x+5 and then find the line with that slope that goes through the point (3,-8).
Make sense?
Then after that, you need to make the lines equal to find the point where they intersect. Then you can use pythagoras to work out the distance between the two points.