x-y=2a - ②

Firstly, ①+② gives x =1+a.

Then ①+② gives y =1-a.

And because x>0, y<0

That is,1+a >; 0， 1-a & lt； 0

Get a>- 1, a> 1

So in general, you can get a> 1

2、x+5 & gt； 0

x & gt-5

x+a & lt； 0

X<［ [ancient names or Latin modern names of animals and plants]

So-5

If there is no solution, -5≥-a

So a≥5

3. From 5x+3y=3 1.

x+y-p=0

Get 2x=3 1-3p.

X=(3 1-3p)/2 is

positive integer

Get 2y=

5p

-3 1

Y=(5p-3 1)/2 is a positive integer.

So 3 1-2p≥2.

5p-3 1≥2

Solution p≤29/2

p≥33/5

So p = 7, 8, 9, 10,1,12, 13, 14.

Because x and y are positive integers.

So p = 7,9