Rachel works at a bookstore. on tuesday, she sold twice as many books as she did on monday. on wednesday, she sold 6 fewer books than she did on tuesday. during the three days rachel sold 19 books. create an equation that can be used to find m, the number of books rachel sold on monday.

Lets start with what we know:

Tuesday (T) = Monday (M) * 2
Wednesday (W) = T - 6
M+T+W = 19

We can rearrange equation 1 to solve for M:

M=T/2

So we insert the above into the other equations, lets choose equation 3:

(T/2) + T + W = 19

If we substitute equation two into the above then we get:

(T/2) + T + (T-6) = 19
T/2 + 2T = 25
T=10.

Now using equation 1:
M = T/2 = 5