Pythagoras theorem: leg 1 squared + leg 2 squared = hypotenuse squared
In the diagram, the triangle has angles 90 and 45. So the other angle in the triangle must be 45 degrees as well. (180 - 90 -45 = 45)
This means it is an isosceles triangle (since two angles are the same), so the two legs have the same length.
So we can say that length of leg1 = x, and the length of leg2 also equals x
Now let's use pythagoras' theorem:
leg1 = x
leg2 = x
hypotenuse = 16
x^2 + x^2 = 16^2
2x^2 = 16^2
2x^2 = 256
x^2 = 128
x = √(128)
x = 8√2