Question 998042
From {{{x^2 + y^2 = 20}}} and 
x - 2y = 0
Solve the second one and substitute into the first one and get
x = 2y so that
{{{(2y)^2 + y^2 = 20}}}
{{{4y^2 + y^2 = 20}}}
{{{5y^2 = 20}}}
{{{y^2 = 4}}}
y = 2, -2
For y = 2, x = 4
For y = -2, x = -4