So, on a coordinate plane, also called a cartesian plane, there is the x and y axis.
Y is up and down, vertically & x is horizontal.
There is positive and negatives in coordinates.
The way the negatives and positives are positioned is like this:
The left hand side of the 'x axis' is negative.
The right hand side of the 'x axis' is positive.
The top part of the 'y axis' is positive.
The bottom part of the 'y axis' is negative.
Whilst plotting, you always start with the x axis. Considering 3 is a positive, you plot it on the right hand side. With your finger, mark out where the 3 is, and slide it down until you get to -7 on the plane.