Draw straight line joining two points

Here's my problem.
I need to draw a "straight" line on a chart from point A to point C, which passes through point B.
I have coords for A and B, but only x coord is available for point C.
How do I calculate "y" of point C?


$a = array( 100, 200 );
$b = array( 200, 176 );
$c = array( 300, '' );

imageline( $im, $a[0], $a[1], $c[0], $c[1], $color );

This line from A to C should pass through B (200, 176), and should be a straight line.
Any ideas?
Thanks

 

Thanks. worked.