Use FOIL (first, outer, inner, last).
Multiply first terms in each factor: 2x times 2x=4x2, and add to the product of the outer terms: 2x times -3=-6x. So we now have 4x2-6x.
Next add the product of the inner term: -6 times 2x=-12x, so we have 4x2-6x-12x=4x2-18x.
Finally add the product of the last terms: -6 times -3=+18:
4x2-18x+18.