Solve ƒ(x) = 2x – 11, what is the value of ƒ(6)

This shows a function of variable x
f(6) just means replace all x's in the function with 6's
so
f(x)=2x-11 
becomes
f(6)=2(6)-11 .   (notice 2x became 2(6) as it is multiplication)
     = 12-11
     =1

Hopefully this was clear for you, good luck

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