By the very definition of rational numbers, two fractions [tex] \frac{a}{b} [/tex] and [tex] \frac{c}{d} [/tex] are equivalent if and only if [tex] ad = bc [/tex]. This is because rational numbers are defined starting from integers via this equivalence relation.
So, you have
[tex] \frac{-2}{3} = \frac{-12}{21} \iff -2\cdot 21 = 3\cdot (-12) \iff -42 = -36 [/tex]
which is clearly false, so the fractions are not the same.
