声明 int response 在圈外。圈内的地方 response = keyboard.nextInt(); 与 int response = keyboard.nextInt() 在循环之外，你再也无法到达它。另外，将选项放置在循环中，以便每次都显示它们。

public static void main(String[] args) {

    Scanner keyboard = new Scanner(System.in);

    int response;

    do {
        System.out.println("What problem do you want to do?");
        System.out.println("1:Celcius to Farenheit");
        System.out.println("2:Slope Formula");
        System.out.println("3:Averages");
        System.out.println("4:Perimeter");
        System.out.println("5:Exit\n\n");

        response = keyboard.nextInt();
        if (response == 1) {
            System.out.println("Degrees Celcius:");
            int degrees = keyboard.nextInt();
            System.out.println(degrees + " degrees Celcius is " + celsToFarenheit(degrees) + " degrees Farenheit.");
        } else if (response == 2) {
            System.out.println("x1:");
            int x1 = keyboard.nextInt();
            System.out.println("y1:");
            int y1 = keyboard.nextInt();
            System.out.println("x2:");
            int x2 = keyboard.nextInt();
            System.out.println("y2:");
            int y2 = keyboard.nextInt();
            System.out.println("The slope is " + slopeFormula(x1,y1,x2,y2));
        } else if (response == 3) {
            System.out.println("First number:");
            int num1 = keyboard.nextInt();
            System.out.println("Second number:");
            int num2 = keyboard.nextInt();
            System.out.println("The average is " + average(num1,num2));
        } else if (response == 4) {
            System.out.println("Length:");
            int length = keyboard.nextInt();
            System.out.println("Width:");
            int width = keyboard.nextInt();
            System.out.println("The perimeter is " + perimeter(length,width));
        }
    }
    while (response != 5);
}

把这个改成这样：

do
{
int response = keyboard.nextInt();
if (response == 1)

如果你移动 keyboard.nextInt() 呼叫内部 do 循环它现在将反复请求用户输入。
您在循环之前有这行代码，所以它只处理一次。