Simultaneous equation: y? =ax，y=2x+ 1

Get 4x? +(4-a)x+ 1=0

So x 1+x2=(a-4)/4.

x 1*x2= 1/4

So (x 1-x2)? =(x 1+x2)？ -4x 1*x2=(a-4)？ / 16- 1=(a？ -8a)/ 16

By chord length formula, l? =( 1+k？ )(x 1-x2)？ =( 1+2? )(a？ -8a)/ 16= 15

Answer? -8a-48=0

That is (a+4)(a- 12)=0.

The solution is a=-4 or 12.

Substitute into the equation: y? =-4x or y? = 12x