need help with 4th order runge kutta with multiple 2nd ode

hi, i would like to ask am i able to use runge kutta for motion equations?

M(d2x/dt2)=Fn(sin θ - uCos θ )

M(d2z/dt2)=Fn(cos θ + uSin θ ) - Mg

Fn,M,θ,u is constant fn/M = 0.866

i tried Mzdoubledot as shown in the code below, however i dont know how to combine Mzdoubledot with Mxdoubledot.

clc;                                               % Clears the screen
clear all;

h=0.1;                                              % step size
thete=30;
g=9.81;
x = 0:h:3;                                         % Calculates upto y(3)
y = zeros(1,length(x)); 
y(1) = 5;                                          % initial condition
F_xy = @(t,thete) 0.866*(cos(thete)+0.5774*(sin(thete)))-g;

for i=1:(length(x)-1)                              % calculation loop
    k_1 = F_xy(x(i),y(i));
    k_2 = F_xy(x(i)+0.5*h,y(i)+0.5*h*k_1);
    k_3 = F_xy((x(i)+0.5*h),(y(i)+0.5*h*k_2));
    k_4 = F_xy((x(i)+h),(y(i)+k_3*h));

      y(i+1) = y(i) + (1/6)*(k_1+2*k_2+2*k_3+k_4)*h;  % main equation
  end

hold on
plot(x,y,'+-', 'Linewidth', 1.5, 'color', 'blue')
xlabel('x')
ylabel('y')
legend('RK4')