Evaluate command not working (properly)

Hello everyone,

I am trying to use the evaluation command in Grasshopper in several occasions, but when I am trying to insert an expression for a variable like x, it isn't recognized, although I have seen this exact expression in numerous tutorials. What should I do?

This particular example is the following one

x*sin(5*x) 

Syntax error: Unknown variable encountered: x

Even when I input a numeric value to it, nothing changes