How would Mycosynth Golem affect cards like Walking Ballista? Say for an example I have Mycosynth Golem and 5 other artifacts on my side of the field. Would I be able to cast Walking Ballista and it will enter the battlefield with 6 +1/+1 counters on it? Or does it not affect it at all?
Yes, cost reduction does apply to X costs. You choose your value for X prior to actually paying the cost, which gives cost reduction effects the opportunity to apply. Controlling Mycosynth Golem and 5 other artifacts will allow you to cast Walking Ballista for X=3 (cost reduction deducts from the total cost, not just the individual X) without paying any mana.
When you cast your Balista spell, you have to choose a value for X. The spell's cost will later be calculated using this value for X. So if you choose X=3, the mana cost of the Ballista spell will be 6. From there you apply first any additional costs, then cost increasing effects, then cost reduction effects (like from affinity), then Trinisphere. With 6 artifacts total under your control, you can choose X=0, X=1, X=2, or X=3, without having to pay mana for the spell. The Ballista will enter with 0/1/2/3 counters. To get 6 counters, X must be chosen as 6, making the Ballista cost 12 mana. So'd need 12 artifacts for affinity to reduce that to 0.
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