x+y=7, x^2 - y^2 = 21
How do I do this?
It is the difference of two squares.
Difference of two squares - Wikipedia, the free encyclopedia
From the difference of two squares we see that
If you expand the right hand side using FOIL the LHS is obtained:
Since we get
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EDIT: If you prefer you can use the substitution method
and solve that linear equation (y^2 cancels) but I think using the difference of two squares is easier