Statistics Probability Distribution..

Hey

I am stuck on this question in the June '04 Edexcel past paper.. I know how to find the answer but I can't get any further so I am stuck.. It's reasonably easy probably.. Here's the question:

3. A discrete random variable X has a probability function as shown in the table below, where *a* and *b* are constants

x - 0, 1, 2, 3

P(X=x) - 0.2, 0.3, b, a

Given that E(X) = 1,7,

(a) Find the value of *a* and the value of *b*

------------------------

So far, I have got 2b+3a = 1.4

Don't know what to do next..(Worried)

Thanks!