Let's break down the question first:
A computer uses 8 digit mantissa and 2 digit exponent
"digit" means we're talking about decimal number system, and not binary(terminology: bits). So, we need not convert any given number to binary.
a=0.052 and b=28E+11
What does this E + 11 mean? It means $* 10^{11}$, or, 11 zeroes after the number 28.
Now let's solve this:
a = 0.052
b = 2800000000000
Standard forms:
a = 0.52 E-1
b = 0.28 E+13
To do a+b, or any operation; we make the exponents equal.
a = 0.0000000000000052 E+13 //Added 14 zeroes after decimal
b= 0.28 E+13
Now, as mantissa is only realised upto 8 digits, a is seen as
a = 0.00000000 E+13
a is as good as 0, so b+a = b+0 = b.
So, b+a-b = b+0-b = b-b = 0.
Solving by making exponents equal the other way around:-
a = 0.52 E-1
b = 28000000000000.0 E-1
b+a = 28000000000000.52 E-1
Now this term minus b:-
28000000000000.52 E-1 - 28000000000000.0 E-1
=00000000000000.52 E-1
Standard form:
0.0000000000000052 E+13
Mantissa is realised only until 8 digits
So, 0.00000000 E+13
=0