can someone please explain how to do this.

set the to equal:

x^2 - 4x+4 = 2x-4
 solve for X

subtract 2x from each side:
x^2 -6x + 4 = -4

subtract 4 from each side:

x^2 -6x = -8

add 8 to both sides:

x^2 -6x +8 = 0

factor the polynomial:

x = 4 and x = 2

using the line equation replace x with 2 and 4 and solve for y

y = 2(2) - 4 = 0
y = 2(4)-4 = 4

so the 2 points the line  crosses the curve is (2,0) and (4,4)

using those 2 points you can calculate the length:
 distance = sqrt((x2-x1)^2 +(y2-y1)^2

distance = sqrt( (4-2)^2 + (4-0)^2)

distance = sqrt (2^2 + 4^2)
 distance = sqrt (4+16)
= sqrt 20 
= 2 sqrt(5)  EXACT LENGTH