c++ - Why does the program not calculate the value correctly?

I have a task. I need to evaluate the expression:

w = (3x + 6x + 9x ... + 90x) -15 , x = 0.03

The calculation of this expression should occur using a while loop.

I wrote the program code, but I get the impression that it does not work correctly.

Please see if my code is working correctly? I am very inexperienced = (

double x = 0.03;
double w, sum;
int counter1 = 0;
while (counter1 <= 90) {
    counter1 += 3;
    w = counter1 * x;
    sum = sum + w;
}
sum = sum - 15;
cout << "Calculation result, w =: " << sum << endl;

Answer

Your problem is the first time through the loop:

sum = sum + w;

sum is uninitialised so it can contain any garbage.

You should always initialise variables:

double sum = 0;

Also, as @formerlyknownas_463035818 comments, your loop goes too far:

while (counter1 <= 90) {
    counter1 += 3;
    ...

your last term will be 93 * x, not 90 * x as you intended.

So it should be:

while (counter1 < 90) {
    counter1 += 3;
    ...