Is 0.641 x 10^3 correct scientific notation

Which of the following lines is parallel to x=4? a- 3y=9 b- y=4 c- x=2 d- x=y answer this as soon as possible xoxo lo ; )

Mindy bought 5/8 pound of almonds and a 3/4 pound of walnuts.which pair of fraction cannot be used to find how many pounds of nuts she bought in all.

Which equation results from adding the equations in this system? x+6y=9 and -x+2y=-15