Each leg of a 45-45-90 triangle has a length of 6 units. what is the length of its hypotenuse?

Let x = the length of the triangle's hypotenuse
using proportions, this is what the equation looks like:

cross multiply to get:

divide both sides by 45 to get 
x=12

Firstly, consider the 45-45-90 ratio:

Length of leg: Length of leg : Length of Hypotenuse
       1              :           1            :          √2

Now let's compare this to our scenario and find the length of the hypotenuse!
       
                    Length of leg: Length of leg : Length of Hypotenuse
                                 1        :           1            :          √2
Scenario=               6        :           6           :        6√2

So technically, multiply the hypotenuse by 6 to follow the 45-45-90 ratio rule!

Hope this helps!