In a parallelogram, opposite angles are congruent.
This means that the four angles of the parallelogram are:
[tex]L = x+40,\quad M = 3x,\quad N = x+40,\quad O = 3x[/tex]
Moreover, the sum of the interior angles of a polygon with n sides is 180(n-2). So, the sum of the interior angles of a parallelogram is 360.
This means
[tex]L+M+N+O = 360 \iff 2(x+40)+2(3x) = 2x+80+6x = 360 \iff 8x = 280 \iff x = 35[/tex]
And since [tex]O=3x[/tex], we have [tex]O=3\cdot 35=105[/tex]
