Question is not specific, so can only give a general answer.
As an example:
If the equation is of the form
log(x)=5
then raise each side to the power of e to give
e^(log(x))=e^5
Since e^(log(x))=x, we have
x=e^5.
The answer is 21 all you had to do was multiply 7 by 3
Answer:
The explicit formula for Arithmetic Sequence is 
Step-by-step explanation:
Given: Foot length & shoe size in AP.
is foot length and n is shoe size.i.e., 
To find: Formula for Arithmetic sequence .i.e., 
From question,

Also, we know that
Now, consider

Thus using formula of
we get,

Therefore, The explicit formula for Arithmetic Sequence is 
Step-by-step explanation:
117+114= 231
360-231= 129
Now find two numbers that add up to 129