You need a way to clear the screen - cg stands for clear graphics.
Exercise 2 draw that square again. Check.
Time to learn a magic shortcut - the turtle knows how to repeat instructions. To draw the square you can simply say
repeat 4 [forward 100 right 90]
Exercise 3 draw a triangle of side 100 using repeat. Check.
Exercise 4 draw a hexagon (that's six sides) of side 80 using repeat. Check.