To find a percentage of a number, multiply by the decimal form of that percentage(20%=0.20). So, take the cost, $430, and multiply by common state sales tax (7% for me)---430 x 0.07 = 30.10 The sale tax rate is 7%.
The slope intercept form is given by

where m is the slope and c is the y - intercept
That is


Hence the equation is