From your equations, you have

x + y = ,
x - y = ,

    or, which is the same,


x + y = 7,    (1)
x - y = 3.    (2)


Now add the equations (1) and (2). You will get

2x = 7 + 3 = 10,   which implies   x =  = 5.


So, the turtle speed in still water is  5 miles per hour.    (<<<---=== at this place I fixed "the slip of my pen").
                                                             (          Thanks to @richwmiller for noticing it !)

Next, from the equation (1) you have  

y = 7 - x = 7 - 5 = 2 miles per hour.


Answer.   The turtle speed in still water is 5 miles per hour.

          The speed of the current is 2 miles per hour.

Please make CHECK on your own.  It is VERY USEFUL and EDUCATIONAL step in such problems.   (And the necessary step, too !)