The following line is parameterized, so that its direction vector is of the form $\begin{pmatrix} a \\ -1 \end{pmatrix}.$  Find $a.$

[asy]
unitsize(0.4 cm);

pair A, B, L, R;
int i, n;

for (i = -8; i <= 8; ++i) {
  draw((i,-8)--(i,8),gray(0.7));
  draw((-8,i)--(8,i),gray(0.7));
}

draw((-8,0)--(8,0),Arrows(6));
draw((0,-8)--(0,8),Arrows(6));

A = (-2,5);
B = (1,0);
L = extension(A, B, (0,8), (1,8));
R = extension(A, B, (0,-8), (1,-8));

draw(L--R, red);

label("$x$", (8,0), E);
label("$y$", (0,8), N);
[/asy]
<|im_start|>system
You are a helpful assistant.<|im_end|>
<|im_start|>user
The following line is parameterized, so that its direction vector is of the form $\begin{pmatrix} a \\ -1 \end{pmatrix}.$  Find $a.$

[asy]
unitsize(0.4 cm);

pair A, B, L, R;
int i, n;

for (i = -8; i <= 8; ++i) {
  draw((i,-8)--(i,8),gray(0.7));
  draw((-8,i)--(8,i),gray(0.7));
}

draw((-8,0)--(8,0),Arrows(6));
draw((0,-8)--(0,8),Arrows(6));

A = (-2,5);
B = (1,0);
L = extension(A, B, (0,8), (1,8));
R = extension(A, B, (0,-8), (1,-8));

draw(L--R, red);

label("$x$", (8,0), E);
label("$y$", (0,8), N);
[/asy]
Please reason step by step, and put your final answer within \boxed{{}}.<|im_end|>
<|im_start|>assistant

user
\frac{3}{5}
The following line is parameterized, so that its direction vector is of the form $\begin{pmatrix} a \\ -1 \end{pmatrix}.$  Find $a.$

[asy]
unitsize(0.4 cm);

pair A, B, L, R;
int i, n;

for (i = -8; i <= 8; ++i) {
  draw((i,-8)--(i,8),gray(0.7));
  draw((-8,i)--(8,i),gray(0.7));
}

draw((-8,0)--(8,0),Arrows(6));
draw((0,-8)--(0,8),Arrows(6));

A = (-2,5);
B = (1,0);
L = extension(A, B, (0,8), (1,8));
R = extension(A, B, (0,-8), (1,-8));

draw(L--R, red);

label("$x$", (8,0), E);
label("$y$", (0,8), N);
[/asy]
train