I just noticed you have two other triangles that do not match the topology of what is in the tutorial.

As a general rule triangles and ngon (polygons with more then four sides) should be avoided. This rule can sometimes be bent. But as a beginner I would suggest you try to stick with quads.

There is a triangle in the above screen grab from the tutorial for instance that would not be a problem there is also an ngon (see the image below) since this tut was targeted for beginner level at a later point in the tutorial I add a edge to make those into a quads.

If it's your own personal project and you are aware of how the model is going to be rendered and used such as will it be deformed or not then you can save some time and effort by leaving an occasional tri or even an ngon in the model but many would argue this is just bad form. I am of mixed opinion on this so I will leave it to you to decide. However, as you are learning I'd strongly suggest to stick to avoiding tri's and ngons.

In studios they will generally prohibit ngons and tri's even if they do not present a problem. There are a number of very good reasons that they have this rule. One of the most obvious is that in a studio many artists may have to work with your model as it moves through the pipeline. If someone had to adjust the model later in the pipeline that innocuous tri or ngon could become a problem.