y=2x²-3x-4
=2(x²-3/2x)-4
=2(x²-3/2x+9/16-9/16)-4
=2[(x-3/2)²-9/16]-4
=2(x-3/2)²-9/8-4
=2(x-3/2)²-41/8
∴顶点坐标为(3/2,-41/8),对称轴为x=3/2
∵a=2>0
∴抛物线开口向上
∴当x