P(X \geq 2n) = 1 - P(X < 2n) = 1 - \sum_{k=1}^{2n-1} P(X = k) = 1 - \sum_{k=1}^{2n-1} p q^{k-1} = 1 - p\sum_{k=0}^{2n-2}q^{k} = 1 - p \frac{1-q^{2n-1}}{1-q} = 1 - (1 - q^{2n-1}) = q^{2n-1}