f and g are different functions of a variable represented by x. f(x) means a function called f is evaluated or defined by what follows the equals sign: 5x-6. x is a sort of placeholder for any value we might want to put into the function. So, for example, if we want to work out f(2) we simply replace x by 2 and work out the result: 5*2-6=10-6=4 so f(2)=4.
The same applies to function g, which has a different definition. For example, g(3)=3^2-5*3+6=9-15+6=0, so g(3)=0.
I hope that's clearer to you now.