How did you get y=-×+c ? then -1=-3+c

As stated, the general equation for a straight line is y = m x + c where m and c are values to be determined.

Apply the gradient formula using x and y values from the two points given to get gradient = -1

m is the gradient, so substitute m = -1 in the general equation to get y = (-1)x + c

All points on a line must satisfy the line equation. Since (3, -1) is a point on the line, we can substitute x = 3, y = -1 into the equation to get

-1 = (-1)(3) + c

We then simplify and solve to get c = 2 and the final line equation y = -x + 2

I see, thank you!