How about asking him to give you two numbers which when multiplied will give you zero.
He would have to admit that one of them has to be zero.
Now point out that is exactly what your second last line in your solution says
You might want to put in the extra line of
therefore x-7 = 0 or x+2 = 0
then x = 7 or x = -2
(are they using the quadratic formula exclusively to solve a quadratic?)
How do you solve the following equation
x2 - 5x = 14
x2 - 5 x - 14 = 0
(x - 7) (x + 2) = 0
x = 7 or x = -2
How do I explain this to my son who was not taught this way???
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