Explaination For Alternate Angles (Angles within "Z")

I extend the line so that can make two trapezium. We know that ∠b+∠c= 180*.
If ∠c=140*, so ∠b= 40* (180*-140*=40*). ∠c and ∠a are on a straight line,
∠b=140*, so ∠a= 40*. ∠a is 40*,then ∠d will be 140*.