(x, y) = (2, 8)

x = 2

y = 8

Plug the values into the system 

y = ax

ax + y = 16

We will get the following system:

2a = 8 

a = 8/2

a = 4

2a + 8 = 16

Indeed if we plug a = 4 into 2a + 8 = 16 we get:

2*4 + 8 = 16

The value a = 4 yields a solution (2, 8) of the system.