Answer:
45 cm
Step-by-step explanation:
Δ ABC is isosceles with AC, the base thus AB = BC
Perimeter = 42 and AC = 12, thus
AB + BC = 42 - 12 = 30, hence
AB = BC = 30 ÷ 2 = 15
Δ BDC is equilateral, thus has 3 congruent sides
The sides each have a measure of 15 cm ( since BC = 15 ), thus
perimeter of Δ BDC = 3 × 15 cm = 45 cm
