Try starting with a value table instead of a graph. For example, if you have 1 ladder bridges, how many feet of steel rods do you use? Since the equation tells you that you need to create a square with the rods to make a ladder bridge, one bridge would have 4 squares. In addition, each square will be 1-foot in length. So every time you add a new segment to your ladder, you add 1 foot to the ladder's length.
To attach a second ladder bridge to the first, how many additional feet would you have to use? Remember that one side of your square is already given from the first ladder bridge.
So now you have
Ladder Length Feet of Rod Used
2 4+3 OR 7
If you are not comfortable with this table method, you can also try...
These types of equations can also be written algebraically, which may help you graph them. Your first square will always have 4 sides, and any remaining bridge segments will use 3 sides. So we have 1(4)+x(3). Notice that I put the x in place of the number of other bridge segments because we are unsure of this value. Let's say we have 2 bridge segments total. We have one that uses four rods (hence the 1*4) and the other uses three rods. We can then show this by using the equation 1(4)+1(3). To solve, using the order of operations to find that 4+3=7. Because we have only used 7 rods, we must try adding another segment. To do this, we would continue to have 1 four-rod segment and 2 three-rod segments. Our equation would be: 1(4)+2(3). Solve algebraically. As you do this, fill in your table as shown above.
Once your table is filled in so that you have used 19 rods, you should have 6 foot of ladder. Now you can graph, where one of the values in your table is your x-variable and the other value is your y-variable. Graph each point within your table and then connect the points.
Hopefully this not only shows you what steps to take but helps you to understand the system of solving these logic problems better. Let me know if there are any questions.