Answer: (a) 675. (b) 247.5.
Variant 9.2.4. On the sides $AB$ and $AD$ of rectangle $ABCD$, points $M$ and $N$ are marked, respectively. It is known that $AN=9$, $NC=39$, $AM=10$, $MB=5$.

(a) (1 point) Find the area of rectangle $ABCD$.

(b) (3 points) Find the area of triangle $MNC$.

![](https://cdn.mathpix.com/cropped/2024_05_06_1f4981519479532effc1g-10.jpg?height=428&width=603&top_left_y=1134&top_left_x=419)