x(3) is a function "x" being applied to the number 3, because "three times x" is 3x. Of course context would help resolve this if you have a function named x o

x(3) is a function "x" being applied to the number 3, because "three times x" is 3x. Of course context would help resolve this if you have a function named x or not, or a variable named x or not, and if you have both a function and a variable named x, well, you worked for your confusion and you have obtained it; congratulations. :)