For the first question you posted. Get a piece of graph paper and draw your x and y axis, then plot each point on the graph paper. The points will plot a line. Using the slope of the line (dy/dx) and the x-intercept, you can get the formula for that line.

As for the last question, if x, y, and z are reals, and the statement x < y, x*z < y*z is true if and only if z is positive.

Just take the original formula "x<y", and multiply both sides by z,

if z is zero, then both sides equal zero,

if z is negative, and both x and y are positive, since y would be the greater negative, y would be less than x.

if z is negative, and If x is negative and y is positive then x becomes positive and y becomes negative making y < x.

In the expression there can't be a case where y is negative and x is positive, since that would make the "x < y" statement false.

There's also the cases for negative z where either x or y is zero (and the other number is either positive or negative, which ever makes the original statement true), but I'm sure you'd be smart enough to know what happens in those two.