Перебор

#define _CRT_SECURE_NO_WARNINGS
#include<iostream>
#include<string>
#include<alg.h>

using namespace std;
const double EPS = 1e-4;
const int INF = 1e8 + 13;



double func(double x)
{
	return 2. * x*x - 6 * x - 3;
}

pair<double, double> solve()
{

	double left = -1;
	double right = 3;
	double step = 0.00001;
	double answer = INF;
	double answerPoint = -INF;
	for (double x = left; left <= right; x += step)
	{
		double cur = func(x);
		if (answer == INF)
		{
			answer = cur;
			continue;
		}
		if (cur <= answer)
		{
			answer = cur;
			answerPoint = x;
		}
		else
			break;
	}
	return { answer, answerPoint };
}

int main()
{
	freopen("input.txt", "r", stdin);
	freopen("output.txt", "w", stdout);

	pair<double, double> ans = solve();
	cout << "min = " <<  ans.first << "\nwith x =  " << ans.second << endl;

}