The Singing Hedgehog Guide to:
Algebra - Substitution 4
Here is the final example:
Substitute the given values into the following expression:
a³ - 8b - 4c²             a=2, b=-4, c=-3

As before, start with the "Not thinking" line:
2³ -8x(-4) -4x(-3)²        

now sort out the numbers:
8                     not 6!!!! It's 2x2x2 = 4x2 = 8
   + 32             minus times a minus is a plus
          - 36      do it carefully, (-3)² = +9, -4x9 = -36
                 = 4
Now you need to practise:

downloadable worksheet
(save this file then run it)
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