Let x = price of taco
Let y = price of glass of milk
"The cost of a breakfast taco and a small glass of milk is $5.10" translates to

"The cost for two tacos and three small glasses of milk is $11.80" translates to

We now have the system

Solve for y using the first equation

Use the third equation and substitute it to the second equation

Substitute the value of x to the third equation and we have

Since x is the price of taco, the cost of a taco is $3.50.