Find the point on the line, where x=3.
Find the point on the line, where y=5.

Make points A=(1,2) and B=(4,4).
Make the line connecting them.

Make the line y=-2x+4.
Find the intersection between the two lines

Erase all objects.
For demonstration purpose: 
Make two sliders a and b.
Make the line y=ax+b
Make a textbox showing the equation.

a = 1.5 (slider from -5 to 5, step 0.1)
b = -3 (slider from -5 to 5, step 0.1)
c: y = a x + b
text1: "My line is: " + c + ""