C++ :: Lagrange Interpolation Formula - Substitute Values Into Power

I did a function for lagrange interpolation formula. However i`m stuck on how do i substitute values into the power. Here is my code snippet :

for(int i=1;i<getSecretShare+1;i++) {
//calculateValue=getSecret+(10*p)+(2 * pow(p,2));
int storeThreshold=getThreshold;
calculateValue=getSecret+(10*i)+(2 * pow(i,2));
calculateFinalValue=calculateValue % 17 ;
cout<<"the calculated value is "<<calculateFinalValue<<endl;
}

The formula that i want to implement is that a + a1x^1 + a2x^2 + a(t-1) ^t-1

I have gotten the substitution of the x value out. However, i am stuck on how do i calculate the values according to the threshold which the user keys in. For example if the user keys in 3.

It will into account the no. of shares to generate which is for example 5 in this case, then it will then take into account the threshold keyed in , which is 3, i will then calculate using the formula using the no. of shares and also threshold.
if the threshold is 3 and the number of shares to generate is 5. I should have the following:

y(1) = 13 + a1(1)^1 +a2(1)^2
y(2) = 13 + a2(2)^1 +a2(2)^2

the power depends on the threshold which is t-1, 3-1 =2

So I will have to generate two a values based on that and also calculate them according to the power that is dependent on the threshold the first value being to the power of 1 and the 2nd being to the power of 2 if the threshold is 4.

Then I will generate 3 a values and then calculate them :

y(1) = 13 + a1(1)^1 + a2(1)^1 + a3(1)^3

and so on..