I got a Doubt that what if the spring has mass
I found the following question

When one end of a spring (of mass m and length l ) is pulled with velocity V1 and other and with velocity V2 then velocity of a point on spring at a distance x from first end is given by formula  

V1+ (x/l)×V2

I tried to prove this but I didn't get any idea
I tried to by putting x=0, x=l the equation satisfied for those...
My sir said that velocity varies linearly with distance x but I didn't understand why

Please help me with this