From the equation y=x we can :
Replace x in the original equation (given in the question) and solve for x.
Do the same for y, except replace with x.
You should get the coordinates of intersection as (a/4,a/4)
Input the these into dy/dx and solve :
Simplify to -1.
Construct an equation :
y-a/4 = -1(x-a/4)
Simplify to answer in markscheme.