help me please i don't uderstand

Answer:
(a) d = 2.1 (b) a = 8.7
Step-by-step explanation:
[tex]d^{2}+d^{2} = 3^{2}[/tex]
[tex]2d^{2} = 9\\d = \sqrt[2]{9/2} = 2.1[/tex]
The hypotenuse = 5*2 = 10
[tex]5^{2} + a^{2} = 10^{2}\\a^{2} = 100 - 25 = 75\\a = \sqrt{75} = 8.7[/tex]