Since, the angle is right angle. ‌∴‌ Homogenising, ‌x2+y2=4(‌
y−3x
c
)2 ‌⇒c2(x2+y2)=4(y2+9x2−6xy) These lines are perpendicular. ‌∴‌ coefficient of ‌x2+‌ coefficient of ‌y2‌=0 ⇒c2−36+c2−4‌=0 ⇒2c2‌=40 ⇒c2‌=20