The two equations you need are:

2*pi*radius = circumference

pi *(radius)^2

radius is half the diameter

so before the increase in circumference occured, the circumference was

2*pi*25 cm = 50 * pi cm

increasing this by 5 cm gives us

new circumference = 50*pi + 5

using this to find the radius

50*pi + 5 = 2*pi*radius

solving for the radius by dividing both sides of teh equation by 2*pi gives

new radius = (50*pi + 5)/(2*pi)

doubling to give diamter

new diamter = (50*pi +5)/pi

change in diameter = new diamter - old diamter

= (50*pi +5)/pi - 25 cm

new cross section area = (new radius ^2) * pi

old cross section area = (old radius ^2) * pi

difference in cross section area = new cross section - old cross section

That should get you through it, year 7 maths can be tough!!!University gets much worse!!!