Question 969821
4+(7xn) when n = 3

Plug in 3 into n. 
Now, it is unclear whether you are using x as a variable or as a multiplication sign. Therefore, I shall do both.

===
x as a variable:
4+(7*3*x)
ANSWER: 4+21x
===

===
x as a multiplication sign:
4+(7x3)
4+21
ANSWER: 25
===