Question

If x-y=10 And x^2-y^2=40 then y=
a)14
b)7
c)4
d)-3
e)-6

Answer 1

Plug them in:

x-14=10
x=24

24^2-14^2=40? Nope.

x-7=10
x=17

17^2-7^2=40? Nope.

x-4=10
x=14

14^2-4^2=40? Nope.

x-(-3)=10
x+3=10
x=7

7^2-(-3)^2=40? Yup.

x-(-6)=10
x+6=10
x=4

(-6)^2-4^2=40? Nope.