The area of the rectangle is 6 ×8 = 48 square units, so the unshaded area is

1 4

×48 = 12

square units. Therefore

(

1 2

×x ×2) + (

1 2

×(6-x) ×8) = 12

, that is, x + 24 - 4 x = 12, so x=4.