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