The amount she charged per necklace is $5.25
A linear equation is represented by the equation y = mx + b
where m is the slope and b is the y intercept.
In this scenario, the slope of the equation will indicate the amount charged by Sandra on each necklace.
P = 7.5n - ( 2.25n + 15)
solving the equation
P = 7.5n - 2.25n - 15
P = 5.25n - 15
n is the number of necklace
m = $5.25
This means each necklace will cost $5.25
Therefore , The amount she charged per necklace is $5.25