Question 131203
SORRY BUT A=10 & B=3 ARE NOT A SOLUTION.
a-b=7 or a=7+b
now replace a with (7+b) & solve for b.
7+b-4b=-7
-3b=-14
b=-14/-3 
b=14/3 answer.
a-14/3=7
a=7+14/3
a=(21+14)/3
a=35/3 answer.
proof:
35/3-4(14/3)=-7
35/3-56/3=-7
-21/3=-7
-7=-7