Question 379054
In this expression, x is a variable which represents an unknown number. For algebra problems in which we know the value of x, we can replace x with this number.

Many times, you'll see something like f(x) (or g(x), h(x), etc.). This does not mean f times x. Instead, f(x) is a function in terms of x. For example, if f(x) = 4x + 3 and we want to find the value of f(10), we can replace x with 10 to obtain f(10) = 4*10 + 3 = 43.