Explanation:
ABC and ADE are similar triangles.
that means they have the same angles, and there is one scaling factor for all sides from one triangle to the other.
so, we see that AB = 2, AD = 2+1 = 3
the scaling factor f from AB to AD is then
AB × f = AD
2 × f = 3
f = 3/2
the same scaling factor now applies between BC and DE (= x).
BC × f = x
1 × 3/2 = x
x = 3/2 = 1.5