noun
laundry detergent powder
Refers specifically to powdered detergent used for washing clothes. Less common in modern usage where liquid detergents are prevalent, but still understood.
洗濯粉を入れて洗濯機を回す。
I put in laundry detergent powder and run the washing machine.
粉末洗剤 is a more modern and technical term for powdered detergent, while 洗濯粉 is a traditional, everyday word.
Compound of 洗濯 (laundry) and 粉 (powder). The reading せんたくこ is a straightforward combination of the on'yomi of 洗濯 and the kun'yomi of 粉.