B=6 i believe! if you use the pythagorean theorem (a^2+b^2=c^2) then you should be able to turn 8^2 and 10^2 into 64 and 100, respectively. 100-64=36 and the square root of 36 is 6. you can plug it in by calculating 8^2 + 6^2, which equals 100! hope this wasn't too confusing XD
X=4
2(3)+2(5)=16
2x+2(3x-8)=16
2x+6x-16=16
8x=32
X=4
Y=10x+15
X= 0
Y= 15
Hope this help