4a)
Looking at the figure,
AD and BE are parallel lines
AE and BD are transversals
Thus,
angle DAE is congruent to angle BEA because they are alternate angles(They lie in similar positions but on alternate sides of the transversal
Also,
angle ADB is congruent to angle EBD because they are alternate angles(They lie in similar positions but on alternate sides of the transversal.
Angle ACD is congruent to angle BCE
Recall, 2 triangles are similar if at least, 2 of their corresponding angles are congruent. In this case, 3 corresponding triangles are congruent. Thus,
triangle ADC is similar to triangle EBC by AA(angle angle) postulate
4b) If 2 triangles are similar, it means that the ratio of their corresponding sides is equal. Thus,
AD/EB = DC/BC = AC/ CE
From the information given,
AD = 2
AC = 4
EB = 5
Thus,
2/5 = 4/CE
By cross multiplying,
2 * CE = 5 * 4
2CE = 20
CE = 20/2
CE = 10