No first derivative term means the wronskian is constant everywhere not just computable at 0

For $a \in \mathbb{R}$, let $y_{1}(x)$ and $y_{2}(x)$ be solutions of the differential equation $y'' + (e^(x^{2}) + \cos x)y = 0$ such that $y_{1}(0) = 3, y_{1}'(0) = -1, y_{2}(0) = -5, y_{2}'(0) = a$. Suppose $W(y_{1},y_{2})(x)$ denotes the Wronskian of $y_{1}$ and $y_{2}$. If $W(y_{1},y_{2})(1/2) = 4$, then the value of a is