1) Use the formula they gave you for the arc length s having radius r and subtending angle a: s = ra. Assuming "pie" really means "pi" (or ), plug in "12pi" for "s" and "30 degrees" for "a". Solve for "r".
2) A length cannot have squared units. Did they perhaps actually give you the area of the sector, rather than the length of the arc?
If so, then use the formula they gave you for the area A of the sector having radius r and subtending angle a: A = (a/2)(r^2). Plug "18pi" in for "A" and "45 degrees" in for "a". Solve for "r".
If you get stuck, please reply showing how far you have gotten. Thank you!