Write an equation for a in the diagram giving reasons. Solve this equation to find the angles a and B giving reasons.
For trapezoids, adjacent angles of the non-parallel sides are supplementary (they add up to 180 degrees). So you know that:
$\displaystyle (4a + 7) + 2a + 11 = 180$
Once you solve for a, you can figure out B by using the same property as above but with the other non-parallel side: $\displaystyle B + 3a = 180$