Answer: 
-13            
  ——— = -0.20635  
  63             
Step-by-step explanation:
Step  1  :
            7
 Simplify   —
            9
Equation at the end of step  1  :
  4    7
  — -  —
  7    9
Step  2  :
            4
 Simplify   —
            7
Equation at the end of step  2  :
  4    7
  — -  —
  7    9
Step  3  :
Calculating the Least Common Multiple :
 3.1    Find the Least Common Multiple
      The left denominator is :       7  
      The right denominator is :       9  
        Number of times each prime factor
        appears in the factorization of:
 Prime  
 Factor   Left  
 Denominator   Right  
 Denominator   L.C.M = Max  
 {Left,Right}  
7	1	0	1
3	0	2	2
 Product of all  
 Prime Factors  7	9	63
      Least Common Multiple:
      63  
Calculating Multipliers :
 3.2    Calculate multipliers for the two fractions
    Denote the Least Common Multiple by  L.C.M  
    Denote the Left Multiplier by  Left_M  
    Denote the Right Multiplier by  Right_M  
    Denote the Left Deniminator by  L_Deno  
    Denote the Right Multiplier by  R_Deno  
   Left_M = L.C.M / L_Deno = 9
   Right_M = L.C.M / R_Deno = 7
Making Equivalent Fractions :
 3.3      Rewrite the two fractions into equivalent fractions
Two fractions are called equivalent if they have the same numeric value.
For example :  1/2   and  2/4  are equivalent,  y/(y+1)2   and  (y2+y)/(y+1)3  are equivalent as well.
To calculate equivalent fraction , multiply the Numerator of each fraction, by its respective Multiplier.
   L. Mult. • L. Num.      4 • 9
   ——————————————————  =   —————
         L.C.M              63  
   R. Mult. • R. Num.      7 • 7
   ——————————————————  =   —————
         L.C.M              63  
Adding fractions that have a common denominator :
 3.4       Adding up the two equivalent fractions
Add the two equivalent fractions which now have a common denominator
Combine the numerators together, put the sum or difference over the common denominator then reduce to lowest terms if possible:
 4 • 9 - (7 • 7)     -13
 ———————————————  =  ———
       63            63  
Final result :
  -13            
  ——— = -0.20635  
  63