Given that

$$M(2a,5b),N(4a,9b),P(6a,5b)andQ(4a,0)$$

So:

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

$$height=5a$$

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

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

$$Area=10ab$$