hello dear..
here ,we have to apply the concept of
( SS1 = T2 )

(1) (y2 - 4ax ) * (k2 - 4ah) = [ yk - 2a (x+h) ]2
above is the pair of straight line which is the tangent from point p to the parabola...
angle a and b with axis of parabola,find locus of p for a+b=900
a = slope of one tangent
b = slope of second tangent
m1m2 = 1 from the given condition ,tak tan of (a+b)
find the value of m1m2 from the equation 1,you will get the answer