The demand and supply functions for two interdependent commodities are given by QD$1=40-5$P$1-$ P$2$

QD$2=50-2$P$1-4$P$2$

QS$1=-3+4$P$1$

QS$2=-7+3$P$2$

where QDi, QSi and Pi denote the quantity demanded, quantity supplied and price of good i$,$ respectively.

$1)$ Determine the equilibrium price an

d quantity for this two$-$commodity model. $2)$ Are these goods substitutable or complementary?