X=first, (fixed) rate
t=time of call
7minutes is $10, 4minutes is $6.40
Plug them in, isolate one variable, set them equal to each other, and solve for the remaining variable,
-6y= -6y, -3y= -3y
pick one original equation, plug in your new value, and solve for the remaining variable
Fixed first minute is $2.80, with $1.20 per each additional minute
I gave you the equations so that you could have them to help you on later problems. And I hope the directions helped.