Because you have to go in the order of PEMDAS, parenthesis, exponents, multiplication, division, addition, and subtraction. From what you are saying, 'why you must simplify 3 to the second power first' it seems that you have no parenthesis, so you move onto what is next, which is exponents, in this case, 3 to the power of two.
S.A = (2×area of base) + area of lateral faces
Area of lateral faces = area of base × height
Area of lateral faces = 19×4×8=608
S.A = (2×19×4)+608
=760
You have to add all the common terms together.
The answer to the question
64 lb = 29.0299 KG
Rounding to the nearest hundredth, it's 29.03 KG