Hard Problem

In order for a system to have infinitely many solutions, the equations must represent the same coefficients and constants. 

We have the equations:

2x + 5y = - 8

6x + 15y = 16 + a

Multiplying the first equation by 3 gives us:

6x + 15y = -24

6x + 15y = 16 + a

In order for this system to have infintely many solutions, 16 + a = -24

This gives us -40 for a. 

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