So you can set this up as a linear equation
y = mx + b
y represents the total amount earned which in this case is $155.
b in this case is the amount earned without any deliveries, $71.
m is the rate/ earning per delivery which is $6
So you can write it as
$155 = $6x + $71
Solve for x
$84 = $6x
14 = x
x = 14
This means that Ben did 14 deliveries yesterday.