1. The problem is about representing multiplication explicitly in STACK notation. 2. In mathematics, multiplication is often implied by writing variables and numbers side by side, for example, $3x$ means $3 \times x$. 3. However, in STACK, the multiplication symbol must be explicitly written as $*$, so we write $3*x$ instead of just $3x$. 4. This means to multiply 3 and x explicitly, we must type $3*x$ or $x*3$, which both represent the same multiplication but with the explicit operator. 5. Simply writing $3x$ without the $*$ symbol is not valid in STACK because STACK requires the operator for multiplication. 6. Conclusion: To indicate multiplication in STACK, always use the $*$ symbol explicitly between numbers and variables.