设抛物线y^2=2px
代入直线得：4x^2+4x+1=2px
x^2+(1-p/2)x+1/4=0
弦的端点为(x1, y1), (x2, y2)
弦长的平方=(x1-x2)^2+(y1-y2)^2=(x1-x2)^2+(2x1-2x2)^2=5[(x1+x2)^2-4x1x2]
=5[ (1-p/2)^2-1]=15
解得p=-2, 6