1. The problem is to understand why the error occurred when trying to define or manipulate the expression $$E := 3 \cdot x \cdot y^3 - \tan(2 \cdot y) = 5(x^2 + 3x)^3 + 5y$$ and how to fix it. 2. The error message "invalid subscript selector" and "unexpected result from Typesetting" typically happens in symbolic computation software when the syntax or structure of the expression is incorrect or unsupported. 3. One common cause is using the assignment operator ":=" with an equation containing an equals sign "=", which is not a valid assignment but an equation definition. The software expects either an expression or a function definition, not an equation. 4. To fix this, separate the equation from the assignment. For example, define the left and right sides separately or use an equation object if supported. 5. Alternatively, if you want to define a function or expression, avoid using "=", and instead write: $$E := 3 \cdot x \cdot y^3 - \tan(2 \cdot y) - 5(x^2 + 3x)^3 - 5y$$ which represents the expression set to zero. 6. Then you can work with $E$ as an expression, solve $E=0$, or plot it. 7. Always check the syntax rules of the software you are using to distinguish between assignment, equation, and expression. Final answer: The error happened because the expression mixes assignment ":=" with an equation "=", which is invalid. Fix by defining $E$ as an expression without the equals sign, for example: $$E := 3 \cdot x \cdot y^3 - \tan(2 \cdot y) - 5(x^2 + 3x)^3 - 5y$$