Answer: 25.2361459992ft
This question can be solved by a trigonometric equation. Shadow formed when something with height like a tree, so it can be pictured as a vertical line. Shadow is a length of and can be pictured as a horizontal line.
In trigonometric, to find the vertical line length with horizontal line you need tan function. The sun angle will be used in the function, so the calculation would be: tan(31) x 42ft = 25.2361459992ft