Question 30In the accompanying figure, segments $AB$ and $CD$ are parallel, the measure of angle $D$ is twice that of angle $B,$ and the measures of segments $AD$ and $CD$ are $a$ and $b,$ respectively. Then the measure of $AB$ is equal to: $\tfrac12a+2b$$\tfrac32b+\tfrac34a$$2a-b$$4b-\tfrac12a$$a+b$
