(a) The instantaneous speed when t=6 sec is the slope of the curve at that point.
Did you draw the red line on the graph ? That's absolutely the way to do part-(a), but the line you draw should kiss the curve at the next point ... the point on the curve where t=6, not the point where t=5.
Draw another line that just touches the curve where t=6. Then, as well as you can, measure the slope of that line. You'll have to do some measuring and some estimating. Mark off a section of the line you draw. Then measure how much horizontal distance that section covers, and how much vertical distance it covers.
The slope of the line is (vertical distance) divided by (horizontal distance). Its unit is meters/second.
11. (b) Finding the average speed is much easier than finding the instantaneous speed. The average is just the slope of a line between the ends of the graph, and you don't care how the graph twists and turns between the start and end points.
The point at the beginning is (0 sec, 0 meters) .
The point at the end is (10 sec, 200 meters) .
Average speed = (distance covered) divided by (time to cover the distance) .
