If y=5/2(e^(x/5)+e^(-x/5)), then prove that y''=y/25

Ok can someone just check with me.. I'm pretty close to the answer but i don't know what I did wrong ..

the final answer of y''=y/25.. I have

[e^(x/5)+e^(-x/5)]/[e^(x/5)/25+e^(-x/5)/25]

Don't know what happened to the 5/2 in y and.. the denominator is almost there but not quite. ^.^

Please explain your method to proving this. Thank you so much!