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

100 - 10P = 50+10p

Subtracting 100 from both side you will get

- 10P = 10P +50 -100

- 10P = 10p -50

Now subtracting 10P from both sides you will get

- 10P - 10P = -50

-20p = -50

Dividing both sides by -20 you will get

-20/-20 P = -50/-20

P = 2.5 >>answer

Checking:

If P= 2.5 then

Qs = 10(2.5)+50

=25+50

=75

Qd = 100 - 10(2.5)

=100 - 25

= 75

Qs = Qd which is 75 = 75

100 - 10P = 50+10p

Subtracting 100 from both side you will get

- 10P = 10P +50 -100

- 10P = 10p -50

Now subtracting 10P from both sides you will get

- 10P - 10P = -50

-20p = -50

Dividing both sides by -20 you will get

-20/-20 P = -50/-20

P = 2.5 >>answer

Checking:

If P= 2.5 then

Qs = 10(2.5)+50

=25+50

=75

Qd = 100 - 10(2.5)

=100 - 25

= 75

Qs = Qd which is 75 = 75