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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There's no immediate version of mullw. Multiply by a variable and you get mullw; multiply by a constant and you get mulli, which you saw in the intro. (And one special constant, a power of two, skips even mulli; that's the next lesson.)
Your task
Write func_8037dd24 so it compiles to the mullw above.