Question 155851
Watch your units.
It'd be tough for a 6 foot patient to use a 6 inch crutch.
You need to convert the height from feet to inches to use the formula. 
6 feet=72 inches
{{{y=0.72x+2}}}
Let's change the equation to make it more meaningful
{{{CH=0.72(PH)+2}}}
where CH stands for crutch height (in inches) and PH, patient height (in inches).
a) PH=72
{{{CH=0.72(PH)+2}}}
{{{CH=0.72(72)+2}}}
{{{CH=53.84}}}
b)CH=40
{{{CH=0.72(PH)+2}}}
{{{40=0.72(PH)+2}}}
{{{0.72(PH)=38}}}
{{{PH=52.78}}}
c) For every inch of PH, CH changes by 0.72. 
d) {{{CH=0.72(PH)+2}}}
At 30 inches,
{{{CH=0.72(30)+2}}}
{{{CH=23.6}}}
At 100 inches,
{{{CH=0.72(100)+2}}}
{{{CH=74}}}
{{{drawing( 300, 300, -10, 110, -10, 90,grid( 1 ),circle(100,74,1.8),circle( 30, 23.6, 1.8 ),green(line( 30,23.6,100,74)))}}}