General desolve and conditions questions

[Rich]

Hello, engineering student here and recently purchased the prime after having a 48G and 50G for a few years. I believe I am a prime candidate for the calculator, being that I am a student. : )

When using desolve I am getting two different results with different variables.

1. desolve(diff(y)+y=5 AND y(0)=3,y returns -2*x^-x+5
2. desolve(diff(x)+x=5 AND x(0)=3,x [[]]

Any reason why i cannot use syntax 2 for a similar result?

Also, is there is anyway I can set a condition when integrating with conditions?

for example:

3. f(x)= a/(2^x2 - 2*b) where b>0 ?

I have tried piecewise and the result does not look like it is accounted for?

Thank you so much in advance.