The function is
f(x) = 12/(4x+2)
Note the use of parenthesis to indicate that 12 is dividing over all of "4x+2"
Plug in x = -1 and use PEMDAS to simplify
f(x) = 12/(4x+2)
f(-1) = 12/(4(-1)+2) ... every x is replaced with -1
f(-1) = 12/(-4+2)
f(-1) = 12/(-2)
f(-1) = -6
Answer: -6