In order to find the market equilibrium set Qd= Qs which is equal to

100 - 2P = 4P - 80

Subtracting 100 from both side you will get

- 2P = 4P - 80 -100

- 2P = 4p -180

Now subtracting 4P from both sides you will get

- 2P - 4P = -180

-6p = - 180

Dividing both sides by -6 you will get

-6/-6 P = -180/-6

P = 30 >>answer

If P= 30 then

Qs = 4(30)-80

=120-80

=40 >>answer

Qd = 100 - 2(30)

=100 - 60

= 40

Qs = Qd which is 40 = 40

100 - 2P = 4P - 80

Subtracting 100 from both side you will get

- 2P = 4P - 80 -100

- 2P = 4p -180

Now subtracting 4P from both sides you will get

- 2P - 4P = -180

-6p = - 180

Dividing both sides by -6 you will get

-6/-6 P = -180/-6

P = 30 >>answer

If P= 30 then

Qs = 4(30)-80

=120-80

=40 >>answer

Qd = 100 - 2(30)

=100 - 60

= 40

Qs = Qd which is 40 = 40