Is 2.1 x 4.9 x 7.2 = 105.84 a reasonable estimate

No.

You'd estimate this as 2 × 5 × 7.2 = 72.

You only keep the decimal place in 7.2 because the othe factor is 10 and it helps reduce the error.

The actual value is 2.1 × 4.9 × 7.2 = 74.088, nowhere close to 106.

correct answer is infinitely many solutions

step-by-step explanation:

2(2x + 3) = -4 + 2(2x + 5)

solving the above equation:

4x + 6 = -4 +4x +10

combining like terms

4x -4x = -4+10-6

0=0

since both sides are 0=0, we end up into true statement it means it has infinite many solutions.

so correct answer is infinitely many solutions