Step one: Find the longest side length listed.

Step two: Add up the other two sides (the two shortest sides).

Step three: Is the sum of the two shortest sides shorter than or equal to the longest side?

Step four: If so, it is not a triangle. If the sum is shorter, the three sides will not connect. If it is equal, it will just form a straight line.

I'll do the first problem for you as an example

Step one: The longest side is 8.

Step two: 5 + 3 = 8

Step three: The two numbers are equal.

Step four: It forms a straight line, so it is not a triangle.

Hope that helped! If you need a more extensive explanation, let me know and I'll be glad to give it.