Answer: Decrease, decrease
Explanation: Substitute goods are those goods that are used in place of each other. When the price of a substitute good falls, it becomes more attractive to the consumers. Here, wood planks and wood beams are substitutes to each other. So, when price of wood plank falls, demand for wood beams will decrease, shifting the demand curve to the left. As a result, of this the equilibrium price of wood beams and the quantity of wood beams will also decrease.