That third angle is 89o obtained by subtracting the other two angles. 180 – 40 – 51 = 89.
If we call it 90o and use tangent of 40o we estimate the other adjacent side about 11/0.839 = 13.1.
Still calling that angle 90o we then calculate the area as (1/2)(13.1)(11) and that equals 72.1.
That isn't exact because we made some approximations along the way, but it's close to right.
The closest choice is 73.1 km2 and I'll bet a yankee dime that's the correct answer.