please help, thank you very much.

the first one is true




if the angles of a right triangle are 90,45,45

the legs are equal. so if AC is 13, Back will be 13, too.


now you can get help from Pythagoras theorem :
[tex]c {}^{2} = {13}^{2} + {13}^{2} \\ c {}^{2} = 169 + 169 = 338 \\ c = \sqrt{338} = 13 \sqrt{2} = 18.38[/tex]




good luck