x2-3x=18
x2-3x-18=0
x1,2=[-b±√(b2-4ac)]/2a
x1,2=[-(-3)±√(-32-4(1)(-18))]/2(1)
x1,2=[3±√(9+72)]/2
x1,2=[3±√(81)]/2
x1,2=[3±9]/2
x1= 6
x1= 3
EL 6 NO CUMPLE, PERO EL 3 SI
$${{\mathtt{3}}}^{{\mathtt{2}}}{\mathtt{\,\small\textbf+\,}}{\mathtt{3}}{\mathtt{\,\times\,}}{\mathtt{3}} = {\mathtt{18}}$$
.