Five pounds at 5/2 gets you £17.50 back. (Two and a half times your stake plus your stake back).
£17.50 at 7/4 gets you £48.12 as marje says. The easiest way to calculate winnings at 7/4 is (Stake plus evens plus half plus half). So that's £17.50 + £17.50 + £8.75 + £4.37 = £48.12.
(There's a simple formula to calculate returns for all commonly used odds. 6/4 is stake evens plus a half. 13/8 is stake plus evens plus a half plus a quarter)