prove if n - natural number divide number $34x^2-42xy+13y^2$ then n is sum of two square number prove if n - natural number divide number $34x^2-42xy+13y^2$ where x,y are relatively prime  then n is sum of two square number. 
I don't know what is going on in this exercise. I will be grateful for explanation.

They are asking about Gauss reduction. The given form is positive and of discriminant $−4.$ That means there is an invertible (determinant 1) change of variables in $SL_2 \mathbb Z,$ call it
$$
P =
\left(
\begin{array}{rr}
 \alpha & \beta \\
\gamma & \delta
\end{array}
\right)
$$
 that takes one form to the other, and preserves gcd. If $u=2x+3y, v=3x+5y,$ then $$34u^2−42uv+13v^2=x^2+y^2$$
 Backwards, $$(5x−3y)^2+(−3x+2y)^2=34x^2−42xy+13y^2.$$
Here we are looking at
$$
H =
\left(
\begin{array}{rr}
 34 & -21 \\
-21 & 13
\end{array}
\right)
$$ and
$$
P =
\left(
\begin{array}{rr}
 2 & 3 \\
3 & 5
\end{array}
\right)
$$
Taking 
$$
Q = P^{-1} =
\left(
\begin{array}{rr}
 5 & -3 \\
-3 & 2
\end{array}
\right),
$$
The first displayed line equation $u=2x+3y, v=3x+5y,$ $34u^2−42uv+13v^2=x^2+y^2$ is
$$ P^T H P = I, $$ the second is
$$ Q^T  Q = H. $$ The second ought to make you think of Cholesky, only with integers required. 
It happens that $P$ and so $Q$ are symmetric so the transpose signs are redundant, not important. 
EDIT: Gauss reduction is a step by step procedure intimately linked with the Euclidean algorithm. At each step, we are changing the symmetric matrix in the middle by multiplying on the right by either some
$$
P_j =
\left(
\begin{array}{rr}
 1 & \beta \\
0 & 1
\end{array}
\right)
$$
or
$$
P_j =
\left(
\begin{array}{rr}
 0 & -1 \\
1 & 0
\end{array}
\right)
$$
and keeping the running product $P_1 P_2 \cdots P_r = P.$ The changes on the triple of coefficients go this way:
$$  \langle 34, -42, 13 \rangle, $$
$$  \langle 13, 42, 34 \rangle, $$
$$  \langle 13, -10, 2 \rangle, $$
$$  \langle 2, 10, 13 \rangle, $$
$$  \langle 2, 2, 1 \rangle, $$
$$  \langle 1, -2, 2 \rangle, $$
$$  \langle 1, 0, 1 \rangle. $$
The particular string $P_1 P_2 \cdots P_6 = P$ that accomplishes this is
$$
\left(
\begin{array}{rr}
 0 & -1 \\
1 & 0
\end{array}
\right)
\left(
\begin{array}{rr}
 1 & -2 \\
0 & 1
\end{array}
\right)
\left(
\begin{array}{rr}
 0 & -1 \\
1 & 0
\end{array}
\right)
\left(
\begin{array}{rr}
 1 & -2 \\
0 & 1
\end{array}
\right)
\left(
\begin{array}{rr}
 0 & -1 \\
1 & 0
\end{array}
\right)
\left(
\begin{array}{rr}
 1 & 1 \\
0 & 1
\end{array}
\right) =
\left(
\begin{array}{rr}
 2 & 3 \\
3 & 5
\end{array}
\right)
$$
