For #6 + #8 You just have to double the radius to get the diameter. So you add 6 cm and 6cm to 12cm and you know 8 because it is the same process just not a whole number.
10 is easier than it looks because the formula for the circumference is 2 times pie times the radius or simpler is the diameter times pie. So you times 7 by 2 and then pie which will give you 43.96