504. Solve the system of equations: 2x + 5y = -7, 3x - y = 15.

Solution:

1. From the second equation, express y: y = 3x - 15.
2. Substitute this expression for y into the first equation: 2x + 5(3x - 15) = -7 => 2x + 15x - 75 = -7 => 17x = 68 => x = 4.
3. Substitute the value of x back into the equation for y: y = 3(4) - 15 = 12 - 15 = -3.

Answer: x = 4, y = -3