a) Do you know the formula for cross-price elasticity of demand? It is (Proportional ch. in demand for good A) divided by (Proportional ch. in price of good B)
In this case, good A is Pepsi and good B is Coke.
The proportional change in demand for Pepsi is
The proportional change in price for Coke is to 3 decimal places
Thus the cross elasticity of demand is roughly (+)2.4. Since the value is positive the products are substitutes.
(b) Yes. Try to think about this intuitively. The price of Coke falling would probably lead to some people switching from Pepsi to Coke, since the opportunity cost of Coke has fallen.
Furthermore, you could also say since its value of the cross elasticity of demand is greater than 1 the goods are strong substitutes.
I don't know how advanced your answer must be, but you can also link in the marginal rate of substitution and budget constraint / indifference curve analysis. If you need assistance in doing so, I'll be more than happy to help.