How can I show a real number on a graph?

The graph for the real numbers is the number line.

1) decide on a scale - say for example 1 cm = 1 unit
2) draw and label a number line
3) use the magnitude of the number to determine how far from 0 to place a point
4) use the sign of the number to determine whether to plot it left (for negative) or right of 0

How can I rewrite the expression x+(y+-z)using the associative property of addition?

Associative property tells us that we can add in any order and the parenthese tells us where we are choosing to add first: 

x+(y+-z)=(x+y)+(-z)

what are the properties of real numbers under addition and multiplication?

the set of real numbers with the operations of addition and multiplication is a field. So this system has all the properties of a field with these:
Additive identity: 1
Multiplicative identity: 0

Please explain the difference between real numbers, rational numbers, and irrational numbers

The real numbers are all the numbers on the number line so they include rational and irrational numbers.  Rational numbers are numbers that can be written as fractions and irrational numbers are the numbers that cannot be written as fractions.

represent 40/8 and 1/4 on a number line

40/8 is located at 5 units to the right of 0 and we find 1/4 by dividing the distance between 0 and 1 by 4 equal size segments the placing a point at the closest segment endpoint to 0 

:)