I need help solving the problem below.

For certain values of k and m, the system a + 2b = -3, 4a + 2b = k - a - mb has infinitely many solutions (a,b). What are k and m?

a + 2b = 3

5a + (2+ m)b = k

This system will have infinite solutions when the equations are the same

Multiply the first equation through by 5

5a + 10b = 15

5a + (2 + m)b = k

This will have infinite solutions when m = 8 and k = 15