A Riemann sum approximates the area of a certain region. Usually it's used in calculus, where you use the Riemann sum to approximate the area a curve instead of using an integral to find the exact value, since that tends to take a lot more time and effort.
87 x 73 = 6,351
2
87
x 73
____
1
The answer is 62 B can u plz mark brainliest

Negative multiply negative = Positive
Negative multiply positive = negative

Thus the answer is -4a + 16b - 12c - 12d.