If you have quantity demanded(Qd) and quantity supplied(Qs) then you can calculate equilibrium price and quantity. To find equilibrium price and quantity you have to find equilibrium point(EP).

EP=> Qd=Qs

suppose, Qd=100-2p &Qs=4p-80

EP=>100-2p=4p-80

=>-2p-4p=-80-100

=>-(2p+4p)=-(80+100)

=>2p+4p=80+100

=>6p=180

=>p=180/6=30

so, equilibrium price is 30

and equilibrium quantity is, Qd=Qs

=>100-2p=4p-80

=>100-2.30=4.30-80

=>100-60=120-80

=>40=40

Hence, equilibrium quantity is 40 and equilibrium price is 30.

EP=> Qd=Qs

suppose, Qd=100-2p &Qs=4p-80

EP=>100-2p=4p-80

=>-2p-4p=-80-100

=>-(2p+4p)=-(80+100)

=>2p+4p=80+100

=>6p=180

=>p=180/6=30

so, equilibrium price is 30

and equilibrium quantity is, Qd=Qs

=>100-2p=4p-80

=>100-2.30=4.30-80

=>100-60=120-80

=>40=40

Hence, equilibrium quantity is 40 and equilibrium price is 30.