Question 710243
<pre>
Find the slope of the first line using the slope formula:

m = {{{(y[2]-y[1])/(x[2]-x[1])}}}

where (x<sub>1</sub>,y<sub>1</sub>) = (a,1)

and where (x<sub>2</sub>,y<sub>2</sub>) = (15,6)

m = {{{(6-1)/(15-a)}}}

m = {{{5/(15-a)}}}

Find the slope of the second line using the slope formula:

m = {{{(y[2]-y[1])/(x[2]-x[1])}}}

where (x<sub>1</sub>,y<sub>1</sub>) = (14,7)

and where (x<sub>2</sub>,y<sub>2</sub>) = (a+2,1)

m = {{{(1-7)/((a+2)-14)}}}

m = {{{-6/(a+2-14)}}}

m = {{{-6/(a-12)}}}

Since they are parallel their slopes are equal, so we 
set the two slopes equal:

{{{5/(15-a)}}} = {{{-6/(a-12)}}}

Cross multiply and solve and get a=30

Edwin</pre>