3 rabbits cost £19.70
the second rabbitt cost £2 more than the first
the third rabbitt cost 80p less than the second
what is the cost of the first rabbitt

can someone please tell me the formula

2. Let the rabbits be $\displaystyle U_1, U_2\ and\ U_3$ respectively

You are told $\displaystyle U_1 + U_2 + U_3 = 19.70$

$\displaystyle U_2 = U_1 + 2$

$\displaystyle U_3 = U_2 - 0.8$

Can you use substitution to find U1? (My answer and working are in the spoiler if you wish to check but remember you're only cheating yourself if you don't bother trying)

Spoiler:
$\displaystyle U_3 = (U_1+2)-0.8 = U_1 + 1.2$ this make sense since £2-80p = £1.20

Back to our first equation

$\displaystyle U_1 + (U_1+2) + (U_1+1.2) = 19.70$

You can collect like terms and solve easily

Spoiler:
$\displaystyle 3U_1 + 3.2 = 19.7$

$\displaystyle 3U_1 = 16.50$

$\displaystyle U_1 = 5.50$

I get an answer of £5.50 which follows because we'd get

£5.50 + £7.50 + £6.70 = £13+£6.70 = £19.70

3. Once you're comfortable with the 3 variables method, you can do it quicker this way:
1st = a
2nd = a + 2
3rd = a + 2 - .8 = a + 1.2

(a) + (a+2) + (a+1.2) = 19.7
3a + 3.2 = 19.7
3a = 16.5
a = 5.5