When we substitute a number for a letter, we simply replace the letter with the number.

However, we must remember that 4b means 4 x b.

**Example**

Work out the value of the following expression when a = 4

3a - 7

**Answer**

3a - 7

3a just means 3 x a , so we have

**3 x a - 7**

Change the letter (a) into the number we've been given (4)

**3 x 4 - 7**

Now multiply first - remember the order of BIDMAS (or BODMAS)

**12 - 7 = 5**

Sometimes we have a **negative number to substitute **and then we need to be very careful about the correct signs!

**Example**

Work out the value of the following expression when** a = -4**

3a - 7

**Answer**

3a - 7

3a just means 3 x a , so we have

**3 x a - 7**

Change the letter (a) into the number we've been given (-4)

**3 x -4 - 7**

Now multiply first - remember the order of BIDMAS (or BODMAS)

**3 x -4 = -12**

Finally, subtract 7:

**-12 - 7 = -19**

When working with negatives, it's important to remember the rules!

Two negative signs next to each other means a positive.

One negative and one positive next to each other means a negative.

If we multiply or divide one positive and one negative, we get a negative.

If we multiply two positives or two negatives, we get a positive.

Are you happy with this?