Answer:
x = 20
Step-by-step explanation:
Given BD = CD then Δ BCD is isosceles and the 2 base angles are congruent
∠ CBD = ∠ BCD = 40° , then
∠ BDC = 180° - (40 + 40)° = 180° - 80° = 100° ← sum of angles in triangle
Sum the 3 angles of Δ ACD and equate to 180
20 + 100 + x + 40 = 180
160 + x = 180 ( subtract 160 from both sides )
x = 20
