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.
Work out the value of the following expression when a = 4 and b = -5
3a - 7 - 2b
3a - 7- 2b
= (3 x a) - 7 - (2 x b)
= (3 x 4) - 7 - (2 x - 5)
= 12 - 7 - -10
= 12 - 7 + 10
Did you remember the rules for negative numbers?
One negative and one positive makes a negative.
Two positives or two negatives makes a positive.
These facts will be very important in this activity!!
Are you ready to begin?