How do you find the length of lines on a graph

I need to find the lengths A(2,1) B(6,3) and of B(6,3) C(5,5), the method of how itsdone would be very much appreciated.

Okay so I'm pretty sure you should know what Pythagoras is. It's basically in a right triangle:
a^2 + b^2 = c^2 where c is the longest side.

So basically this is the formula:
sqrt[(x2 - x1)^2 + (y2 - y1)^2]

Let's apply that right now.

sqrt[(6 - 2)^2 + (3 - 1)^2]
sqrt(4^2 + 2^2)
sqrt(16 + 4)
sqrt(20)
2sqrt(5)

So for the first one, the answer is 2sqrt(5), or 4.472135955

Now for the second:

sqrt[(5 - 6)^2 + (5 - 3)^2]
sqrt[(-1)^2 + 2^2]
sqrt(1 + 4)
sqrt(5)

And that's your second answer: sqrt(5) or 2.236067977

Dear use the distance formula for your question:
d= √(x2-x1)^2+(y2-y1)^2

For coordinates A(2,1) B(6,3) the value of d=2(5)^1/2
For coordinates B(6,3) C(5,5) the value of d=(5)^1/2