I'm actually just <u>assuming</u> that you mean arc NO. Proceeding with that...
Angle NMO is an inscribed angle which intercepts arc NO. Angle NPO is also an inscribed angle that intercepts arc NO. Because they both intercept the same arc, both inscribed angles have the same measure. Therefore,
9x - 3 = 4x + 12
Solving for x:
5x = 15
x = 3. Plug 3 in for x in either one of the equations to get that angles NMO and NPO measure
9(3) - 3 = 24°
The rule is that inscribed angles measure HALF of the arcs they intercept, so the measure of arc NO is 48°