You can start by swapping the x- and y-coefficients and negating one. In standard form the x-coefficient must be positive, so it is convenient to choose that.
... 3x +y = ??
The ?? is a constant chosen to make sure the line goes through the given point. If you fill in 2 for x and -1 for y, you see what the constant needs to be.
... 3x + y = 3(2) +(-1) = 5
The equation is 3x + y = 5.
