Answer:
12.8 units
Step-by-step explanation:
Adding a diagonal makes it into 2 of the same right-angled triangles.
In right angled triangles, you can use Pythagoras' therorem which says that a^2+b^2=c^2 where a and b are the shorter sides and c is the diagonal.
This means that 8^2+10^2=diagonal^2=64+100=164
The diagonal is then the square root of 164 which equals 12.806248...
