We have the equation:
6m/n+p=L
We need to solve it for p. For this, we subtract the 6m/n term on both sides:
6m/n + p - 6m/n = L - 6m/n
But 6m/n - 6m/n = 0 (on the left hand side), so:
p + 0 = L - 6m/n
p = L - 6m/n
