First, let's multiply the first equation by two on the both sides:
<span>8x + 7y = 39 /2
</span>⇒ 16x + 14y = 78
Now, the system is:
<span>16x + 14y = 78
</span><span>4x – 14y = –68
</span>
After adding this up in the column:
(16x + 4x) + (14y - 14y) = 78 - 68
20x = 10
⇒ x = 10/20 = 1/2
y can be calculated by replacin the x:
<span>8x + 7y = 39
</span>⇒ 8 · 1/2 + 7y = 39
4 + 7y = 39
7y = 39 - 4
7y = 35
⇒ y = 35 ÷ 7 = 5
In order to make back the money they spent ($1,137.50) plus $1500, they want to make

$2,637.50 total.
They want to make this amount by selling 350 agendas, so divide the amount they want to make by 350 to find the amount per book:

I rounded the answer to the nearest hundredth because the hundredth is the smallest amount we have in USD (the penny.)
So they need to charge at least $7.54 to make back their money plus $1500.
Hope I helped, and let me know if you have any questions :)