I wanna watch a 15year rated movie, am I allowed?

I wanna watch a 15year rated movie, but I am 12years old... My mum is coming with me, but I'm not sure if I'm gonna be allowed for 2 reasons...1:I'm 12, not 15, 2:It's a horror movie in 3-D...
Wanna watch THE FINAL DESTINATION 3D
Can I?
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