Love when my lecturer doesnt explain why he changed the 16 into 4x4. 🤯🤯🤯

Like I get that 4x4 is 16, but he just did it without explanation. Now I'll have to go in and get him to explain it to me in person cause I missed the lecture and couldn't ask live. 🤯🤯🤯
CONFUSED. If anyone knows please do explain it to my idiotic self. This class has me second guessing myself and it was just meant to be a revision class of highschool maths HAHAHAH. 😐