This question is tough. You have to first notice a pattern, as the number of slanted lines inside the triangle increase, the number of total triangles also increase. However this increase is not random, but rather follows a pattern as shown in the working above.
Another method is to count one by one, but in a systematic order so as not to miss out any triangles. I did not do this method as it is highly prone to mistakes. You can start of by counting the number of smallest triangle, then slowly increase its size.