How can I show that the sum of two martingales is a martingale?

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If I have X and Y, two martingales in the same filtered environment, and a and b two constants, how can I show that :

{aXt + bYt} is also a martingale?

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$E(aX_{n+1}+bY_{n+1}|\mathcal{F}_n)=aE(X_{n+1}|\mathcal{F}_n)+bE(Y_{n+1}|\mathcal{F}_n)=aX_n+bY_n$