Trinisphere+Sphere of Resistance / Thorn of Amethyst

[moOnsteak]

I was looking until page 10 and can't find some similar threads so I 'd to ask about this question. .I'm sorry if this thread will be a repost. .

I play Brainstorm for example. .with my opponent control an untapped Trinisphere and a Sphere of Resistance in his field. .How much mana that I must pay to cast Brainstorm??

I think Trinisphere will work first as its layer to change a spell casting cost, and then Sphere of Resistance give an additional cost so it'll be :3::u:. .but sompe people don't agree with me so I'm not sure about it. .am I right or wrong?
Thanks. .

[Nightmare]

Trinisphere is always applied last. Brainstorm costs :2::u:.

[moOnsteak]

--edited-- wrong post
Thank you for your answer. .

[Anusien]

Rule 419.1f tells you that you apply increases in cost, then reductions in cost, then Trinisphere.