Verified answer

P = (2,1), Q = (2,5) e R = (x,4)

[2 1 1] 2 1

[2 5 1] 2 5

[x 4 1] x 4

det = 10 + x + 8 - 5x - 8 - 2

det = -4x + 8

Area = |det|/2

20 = |-4x + 8|/2

-4x + 8 = ±40

-4x + 8 = 40

-4x = 40 - 8

x = -32/4

x = -8

ou

-4x + 8 = -40

4x = 8 + 40

x = 48/4

x = 12

-8 ou 12