1)  5 + t² will always be larger than 3 - t².
2)  15/x² is larger than 15/7x²
3) 1/x will be larger than 1/(-x), only if the values of x take positive numbers.
4)  k² will be larger than the second (k-6)². if k takes only positive numbers.
1)
If we see the first term, 5 is adding the function t² and in the second term,  t² is negative so we can conclude that 5 + t² will always be larger than 3 - t².
2)
Here, we can analyze the independent values. The first term has 15 and the second 15/7 = 2.14. Now, as we have x² in the bottom, the function decrease as x increase. Therefore, the first term 15/x² is larger than the second 15/7x² 
3) 
We can see that the second term is negative, so 1/x will be larger than 1/(-x), only if the values of x take positive numbers.
4)
Here we have the same case, if k takes only positive numbers, the first term k² will be larger than the second (k-6)² because we have 6 substracting. 
You can learn more about quadratic function here:
https://brainly.com/question/10606041
https://brainly.com/question/12672838   
I hope it helps you!