Here you would use something called pythagoreans theorem. Since this is a diagonal line you need to use this. The theorem states that the leg squared plus the leg squared is equal to the hypotenuse squared(the measurement of this length). Okay so one leg measures 6 and the other 3; keep in mind these legs are drawn in to create a perfect right triangle! if it isn't drawn as a perfect right triangle it won't work. Anyways 6 sqaured plus 3 squared is equivalent to 36 + 9 therefore is equal to 45 which is the square of your length, so in order to fnd your length you square root 45 and it comes out to about 6.708
