For the y intercept form or standard form, you need the slope of the line and the y intercept.

standard form: y = mx + b

we already found the slope, m= -1

now we just need the y intercept (b).

to find b you can plug in the slope and one point into the equation and solve for b.

i'm using the point (-5,9)
y= mx+ b
9 = (-) + b
9 = 5 + b
4 = b

now that we have the slope, m, and the y intercept, b, we use those to form the equation of the line.

y = mx+ b
y = -1x + 4
y = -x + 4

if you take the point slope forms that we found above and solve for y they will match the standard form equation.