Maths problem: how to (b), thanks?

(a)
AC/CE = (8+4)/6 = 12/6 = 2
CD/EB = 10/5 = 2
DA/BC = 8/4 = 2
∵ AC/CE = CD/EB = DA/BC
∴ ΔACD ~ ΔCEB (3 sides proportional)

(b)
∠DAC = ∠BCE (corr. ∠s, ~Δs)
∴ ∠DAC = ∠ACE
∴ AD // EC (alt. ∠s equal)

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