How do you determine which place value to use when estimating products?

(08.01)consider the following pair of equations: x + y = −2 y = 2x + 10 if the two equations are graphed, at what point do the lines representing the two equations intersect? (−4, 2) (4, 2) (−2, 4) (2, 4)

Anyone know? will mark brainliest if correct

He mass of a single atom of carbon can be found by dividing the atomic mass (12.01 g) by 6.022 x 10^23. which is the mass of a single carbon atom, correctly written in scientific notation with the correct number of significant figures?