In order to form a line equation from two points I have been told to do following and not to use any other ways.
$$a(9,6) \qquad b(2,-1)\\ \begin{align} x&=9-7t\\ y&=6-7t \end{align}$$
Cancel out the $t$s gives $$x-y=3$$
but when the signs are opposite I don't know what to do such as
$$a(7,6) \qquad b(8,1)\\ \begin{align} x&=7+1t\\ y&=6-5t \end{align}$$
and also
$$a(-1,-2) \qquad b(7,-3)\\ \begin{align} x&=-1+8t\\ y&=-2-1t \end{align}$$
I with some could show me how to work these questions out in this format so I have a reference I would be very grateful.
Hint: The equation $x=7+t$ holds exactly for the same $x$ and $t$ when $5x=35+5t$.