There are several useful comments on this inquiry.  First, "X3" is probably intended to be "X^3" - the '^' (Shift+6) symbol is used for exponentiation.  It is also good to comment that all variables are case-sensitive and fortunately each of the variables is uppercase, so that is fine.  Finally, to solve the equation for a specific value of 'x', replace 'x' with the numeric value in parentheses, which for '2' is '(2)':

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Y=20