3.
Prompt X1
Prompt Y1
Prompt X2
Prompt Y2
(Y2-Y1) / (X2-X1) --> M
Y1 - M*X1 --> B
Disp M
Disp B

4.
ClDrw
FnOff
Prompt X1
Prompt Y1
Prompt X2
Prompt Y2
(Y2-Y1) / (X2-X1) --> M
Y1 - M*X1 --> B
DrawF M*x + B