How can I get the intersection of these lines

[median27]

Homework Statement

Kindly view my 1st attachment. I need help to get the intersection of the highest line and the line extended by a slope from the horizontal line.

Homework Equations

y=mx+b
y-y₁=m(x-x₁)

The Attempt at a Solution

Kindly view my 2nd attachment.
By substitution, I always get the answer -1.87 which is obviously wrong. The answer should be lesser than -5.5.

Thanks in advance.

 

[danago]

The first equation (y = x+105.5) is not quite correct. The gradient does have a magnitude of 1, however the line slopes downwards along the x-axis, therefore the gradient should be -1, not +1 as you have used. See if that fixes the problem.

 

[median27]

thanks, i'll work on it