test -- Sage

a,b,c = var('a b c') solve([a-b==1/5,c==2*a*b,a^2+b^2==1], a, b, c)

[[a == (4/5), b == (3/5), c == (24/25)], [a == (-3/5), b == (-4/5), c ==
(24/25)]]

[[a == (4/5), b == (3/5), c == (24/25)], [a == (-3/5), b == (-4/5), c == (24/25)]]

test