math
posted by carol .
In solving the equation (x + 4)(x – 7) = 18, Eric stated that the solution would be
x + 4 = 18 => x = 22
or
(x – 7) = 18 => x = 11
However, at least one of these solutions fails to work when substituted back into the original equation. Why is that? Please help Eric to understand better; solve the problem yourself, and explain your reasoning.

Eric does not understand the concept
It only works if the product is zero
so expand ....
x^2  3x  28 = 18
x^2  3x  10 = 0
(x5)(x+2) = 0
x = 5 or x = 2
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