Given that

$$ M(2a,5b),\ N(4a,9b),\ P(6a,5b)\ and \ Q(4a,0) $$

So:

$$ base=MP=6a-2a=4a $$

$$ height =5a $$

$$ Area=\frac{1}{2}\times base\times height $$

$$ Area=\frac{1}{2}\times 4a\times 5b $$

$$ Area=10ab $$