If the value passed into assert evaluates to true, nothing happens.
If the value passed into assert evaluates to false, stderr receives an error message and the program is aborted.
Anything non-zero in C++ is evaluated as true.
Anything zero is evaluated as false.
assert(findSum(1, 1) - 2);
findSum(1,1) returns 2.
2 - 2 returns 0.
assert(0) will abort your program, which I presume you don't want.
Change your statements to something like: assert(findSum(1,1) == 2);
Also note the warnings in your code.
In function 'double findSum(const double&, const double&)':
147:8: warning: variable 'sum' set but not used [-Wunused-but-set-variable]
In function 'double findDifference(const double&, const double&)':
153:8: warning: variable 'difference' set but not used [-Wunused-but-set-variable]
In function 'double findProduct(const double&, const double&)':
160:8: warning: variable 'product' set but not used [-Wunused-but-set-variable]
In function 'double findQuotient(const double&, const double&)':
195:8: warning: variable 'quotient' set but not used [-Wunused-but-set-variable]
Your findAverage function is putting the "result" into the third variable.
As you have it now, its return type is void. That means you shouldn't be trying to compare it to 5.
Either change findAverage so that it returns a double: