ok, you have the slope -5, insert that into y=(slope)*X + C, C is not a variable, it is a coefficient.
You get y=-5*x + c
you are also given the (x,y) values, replace x and y with their values.
-8=-5*(-3) + c
now we solve for c.
(-8 = 15 + c) = (c + 15 = -8), all i did swap the right and left sides. That didn't change the equation at all.
now, we want c alone. So, we subtract 15 from but sides.
c + 15 - 15 = -8 -15,
c = -23. now, you can check the answer
y = (slope)*x + c, we now know all the values. y = -8, x = -3, slope = -5, c = -23
insert known values.
-8 = (-5)*-3 + (-23), check that the equation in balanced
-8 = 15 + -(-23)
-8 = -8
FINAL STEP: insert just the slope value and the coefficient into y = (slope)*x +c
y = (-5)*x -8
you can now graph this and find (x,y) coordinates by placing a new value for x and then solve for y. If the equation balances, you are good!!!