Let
b----------> the missing term
we have
\((10x - 4x^{2} )- (7x + b) = 3x - 6x^{2}\)
Combine like terms in the left side
\((10x-7x)+(-b - 4x^{2}) = 3x - 6x^{2}\)
\((3x)+(-b - 4x^{2}) = 3x - 6x^{2}\)
Simplify
\((-b - 4x^{2}) = - 6x^{2}\)
\(b = 6x^{2}-4x^{2}\)
\(b = 2x^{2}\)
therefore
the answer is
the missing term is equal to \(2x^{2}\)
