Use rule solutions

s = Solve[x + 3 == 5, x]

{{x -> 2}}

s[[1]]

{x -> 2}

x + 4 /. s[[1]]

6

t = Solve[x ^ 2 == 9, x]

{{x -> -3}, {x -> 3}}

x + 4 /. t[[2]]

7

eqns = {x + 3 == 4, y - 5 == 2, 3z == 4}

{3 + x == 4, -5 + y == 2, 3*z == 4}

u = Solve[eqns, {x, y, z}]

{{x -> 1, y -> 7, z -> 4/3}}

u[[1]]

{x -> 1, y -> 7, z -> 4/3}

x + y + z /. u[[1]]

28/3

u[[1, 1]]

x -> 1

x + y + z /. u[[1, 1]]

1 + y + z

x + y + z /. {u[[1, 2]], u[[1, 3]]}

25/3 + x

v = Solve[{x ^ 2 + y ^ 2 + z ^ 2 == 9, y == x, y == z}, {x, y, z}]

{{x -> Sqrt[3], y -> Sqrt[3], z -> Sqrt[3]}}

x + y + z /. v

{3*Sqrt[3]}

eqn = x ^ 2 + 2a x + 1 == 0;

sol = Solve[eqn, x]

{{x -> (-2*a - 2*Sqrt[-1 + a^2])/2}, {x -> (-2*a + 2*Sqrt[-1 + a^2])/2}}

sub = eqn /. sol

{1 + (-2*a - 2*Sqrt[-1 + a^2])^2/4 + a*(-2*a - 2*Sqrt[-1 + a^2]) == 0, 1 + (-2*a + 2*Sqrt[-1 + a^2])^2/4 + a*(-2*a + 2*Sqrt[-1 + a^2]) == 0}

Simplify[sub]

{True, True}

Plot[x /. sol, {a, -5, 5}]

-Graphics-

Plot[x /. sol[[1]], {a, -5, 5}]

-Graphics-