Помогите пожалуйста срочно надо

3) $d=a \sqrt{2} =3 \sqrt{2}$
4) $P=5+12+5+12=34$
$S=12*5=60$
5) $13^{2}- (\frac{AC}{2}) ^{2} = (\frac{BD}{2}) ^{2} \\ 169-25= (\frac{BD}{2}) ^{2} \\ (\frac{BD}{2}) ^{2}=144 \\ \frac{BD}{2} =12 \\ BD=24$
6) $2x+y=16 \\ y-x=1 \\ y=1+x \\ 1+x+2x=16 \\ 3x=15 \\ x=5 \\ y=6 \\ BD^{2} = AB^{2} - AD^{2} \\ BD^{2}=25-9 \\ BD^{2}=16 \\ BD=4$
7) $20^{2} = (3x)^{2} + (4x)^{2} \\ 400=9 x^{2} +16 x^{2} \\ 400=25 x^{2} \\ x^{2} =16 \\ x=4 \\ P=3x+3x+4x+4x=3*4+3*4+4*4+4*4=56 \\ S=3x*4x=3*4*4*4=192$
8) $\frac{21}{x}= \frac{10}{6} \\ x= \frac{21*6}{10} =12.6 \\ BC=12.6-6=6.6$