I am not 100% positive but from what I remember learning I think the answer you are looking for on the first question is C.
Answer:
(5 + 3i)(5 − 3i) = 34
Step-by-step explanation:
The multiplication of two identical binomials who have different symbol, can be solved using the factoring case of a difference of squares:

So, in this case:


Remember that 
So: 
We replace on the polynomial:

Answer:
2.- 100
Step-by-step explanation 1:

↑ As we can see, the only common multiple between 4 and 25 that is given to is 100.
Step-by-step explanation 2:

↑ Another way of knowing the answer is by using a fraction solving method. As we can see, the denominator is once again 100.
Hope it helped,
BiologiaMagister