Question

Must m in the equation y=mx+b always be a positive number

Answer 1

Not at all.

'm' turns out to be the slope of the graph of the equation.

If the graph of the equation slopes upward as it goes
from left to right, then 'm' is positive.

If the graph of the equation slopes downward as it goes
from left to right, then 'm' is negative.

Answer 2

'm' is the slope. 

The answer is no. There could be a negative number, which means that the slope would be decreasing, or as known as a negative slope (negative rate of change). <span />