What value in place of the question mark makes the polynomial below a perfect square trinomial? 16x^2+? x+36

An animal shelter has 21 puppies. if the puppies are 28% of the total dog and cat population , how many dogs and cats are in the animal shelter

Can you me find the missing length. i attached an image.

Ariana starts with 100 milligrams of a radioactive substance. the amount of the substance decreases by 20% each week for a number of weeks, w. the expression 100(1−0.2)w finds the amount of radioactive substance remaining after w weeks. which statement about this expression is true? a) it is the difference between the initial amount and the percent decrease. b) it is the difference between the initial amount and the decay factor after w weeks. c) it is the initial amount raised to the decay factor after w weeks. d) it is the product of the initial amount and the decay factor after w weeks.