Solve using substitution.

M = 5p + 8

M = - 10p + 3

The answer is - 1/3.

What you can do is substitute the botton "M" with 5p+8

so it would turn out to be 5p + 8 = - 10p + 3. You then add 10p to each side resulting in 15p + 8 = + 3. You then minus the 8 resulting in 15p = - 5. So you then divide - 5 by 15 resulting in p=-1/3