Answer: the expression is:
s = 5*b = 5*3.4 miles = 17 miles
Step-by-step explanation:
If we define s as the amount of miles that Sandra did and b as the amount of miles that Barbara did.
We know that:
s = 5*b
and we also know that b = 3.4 miles
so we can replace the value of b in the equation above, and find the equation that says the distance that Sandra rode:
s = 5*b = 5*3.4 miles = 17 miles