i can see how they got it but its bloody hard to explain in words but hopefully you can visualise it
the way i did it is move the 0.6 line up through the trapezium until the bottom bit meets with the X line we are supposed to find
this gives you a little right angled triangle with sides of 0.6 and X, using angles between parallel lines you can find out that the top angle is 75 degrees therefore the 3rd angle must be 180-75=15
then you just apply the cos15=(adjacent/hypotenuse)
There are probably other ways of doing it