Hi there!
When given a point of an equation, you need to know that the first number is on the "x" axis and the second number is on the "y" axis. So you should imagine the point you are given like this : (x, y).
Now that this is clear, you can now know replace the values of "x" and "y" in the equation by their value given in your point :
x = -18
y = 7
x + Dy = 13
-18 + 7D = 13
All you have to do now is solve the equation by isolating the "D" :
-18 + 7D = 13
Add 18 on each side of the equation
7D = 31
Divide each side of the equation by 7
D = 4.42857143
D ≅4.43
There you go! I really hope this helped, if there's anything just let me know! :)
