First we will do -3 times everything in the parenthesis.
-3 times -4n = 12n
-3 times 30 = -90
SO,
12n - 90 = -30
+90 +90
12n = 60
divide 12 on both sides;
n = 5
This is your answer,
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C
pentagram cause it’s a pentagonal prism
c = 
using Pythagoras' identity on the right triangle
c = √( 4² + (
)² ) = √16 +
=
= 