I have answered this question. Here's my solution.
Given:
12 rectangular planks
2 sides per plank
Length = 8 ft
Width = 3 ft
Area of a rectangle = Length * Width.
A = 8ft * 3ft
A = 24 ft²
24 ft² x 2 sides of a plank = 48 ft²
48 ft²/plank * 12 planks = 576 ft² is the total area Alex needs to varnish.