We have,
Equation of parabola y2=4ax
Point (x1,y1)=(−6a,0)
Angle is 45o
Slope
m=tanθ=tan45o
m=1
We know that,
y−y1=m(x−x1)
⇒y−0=1(x+6a)
⇒y=x+6a
⇒x−y+6a=0