Triangles ABC and ADE are similar triangles. That means the sides are all scaled by the same factor.
To go from side BC to side DE, you multiply by 1.25. That means 1.25 is your scale factor. Now apply that factor to the bottom side to get your answer.
Side AB is 6. That means side AD is 6 x 1.25 = 7.5. That means x = 7.5 - 6 = 1.5
