Multiply low
PowerPC multiplies two registers with one instruction, mullw rD, rA, rB ("multiply low word"). It keeps the low 32 bits of the product and discards the rest:
mullw r3, r3, r4
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Notice there's no immediate version of mullw. Any time you multiply by a variable, you end up here. Multiplying by a constant takes a different path, which is the next lesson.
Your task
Write mul2 so it compiles to the mullw above.