\setupcolors[state=start]

\definecolor[gridlines][s=0.7]

\startMPinclusions

color mygridcolor; mygridcolor=\MPcolor{gridlines};

\stopMPinclusions

\starttext

\startbuffer[one]

% initialize scale
numeric u; 10u=4cm;

% draw grid
for k=-5u step 1u until 5u:
    draw (-5u,k)--(5u,k) withcolor mygridcolor;
    draw (k,-5u)--(k,5u) withcolor mygridcolor;
endfor;

% draw axes
drawdblarrow (-5.2u,0)--(5.2u,0);
drawdblarrow (0,-5.2u)--(0,5.2u);

% label axes
label.rt(btex $\tfx x$ etex, (5.2u,0));
label.top(btex $\tfx y$ etex, (0,5.2.u));
label.bot(btex $\tfx 5$ etex, (5u,0));
label.lft(btex $\tfx 5$ etex, (0,5u));

%draw the given line and the parallel line
drawdblarrow (-5u,-4.5u)--(5u,0.5u) withcolor blue;
label.bot(btex $\tfx x-2y=4$ etex, (-5u,-4.5u));

%pairs
pair Q, R;
R:=(4u,0u);
Q:=(0u,-2u);

%labels
dotlabel.lrt(btex $\tfx R(4,0)$ etex, R);
dotlabel.lrt(btex $\tfx Q(0,-2)$ etex, Q);

\stopbuffer

\startbuffer[two]

% initialize scale
numeric u; 10u=4cm;

% draw grid
for k=-5u step 1u until 5u:
    draw (-5u,k)--(5u,k) withcolor mygridcolor;
    draw (k,-5u)--(k,5u) withcolor mygridcolor;
endfor;

% draw axes
drawdblarrow (-5.2u,0)--(5.2u,0);
drawdblarrow (0,-5.2u)--(0,5.2u);

% label axes
label.rt(btex $\tfx x$ etex, (5.2u,0));
label.top(btex $\tfx y$ etex, (0,5.2.u));
label.bot(btex $\tfx 5$ etex, (5u,0));
label.lft(btex $\tfx 5$ etex, (0,5u));

%draw the given line and the parallel line
drawdblarrow (-5u,-4.5u)--(5u,0.5u) withcolor blue;
label.bot(btex $\tfx x-2y=4$ etex, (-5u,-4.5u));
drawdblarrow (-4u,5u)--(1u,-5u) withcolor blue;

%pairs
pair P, Q, R;
P:=(-3u,3u);
Q:=(-3u,1u);
R:=(-2u,1u);
draw P--Q--R withpen pencircle scaled 1pt withcolor red;
label.lft(btex $\tfx \Delta y=-2$ etex, 0.5[P,Q]);
label.bot(btex $\tfx \Delta x=1$ etex, 0.5[Q,R]);

%labels
dotlabel.urt(btex $\tfx (-3,3)$ etex, P);

\stopbuffer

\startbuffer[combined]
  \startcombination[2*1]
    {\processMPbuffer[one]}{(a)}
    {\processMPbuffer[two]}{(b)}
  \stopcombination
\stopbuffer

\startlinecorrection[blank]
\midaligned{\getbuffer[combined]}
\stoplinecorrection

\stoptext