3x*4x

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3x means 3 times x right?  4x means 4 times x.  you basically have 3*x*4*x - ok? Multiplication can be rearranged (commutative property) so how about 3*4*x*x which would be 12 (from the 3*4) x^2 (from the x*x). So, 3x*4x = 12x^2

=12x^2