When using convoke to cast a spell with in its mana cost, first choose the value for X. That choice, plus any cost increases or decreases, will determine the spell's total cost. Then you can tap creatures you control to help pay that cost. For example, if you cast Chord of Calling (a spell with convoke and mana cost ) and choose X to be 3, the total cost is . If you tap two green creatures and two red creatures, you'll have to pay .