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