Setting Up Non-Linear Differential Equations Symbolically

I am trying to solve a set of non-linear differential equations in MATLAB. When I hit run, I get: "Warning: Unable to solve symbolically. Returning a numeric solution using vpasolve."

My two equations are:

tau=I*theta_dbl_dot

-tau=m*g*l*sin(theta)

Here is my code:

syms tau In theta_dbl_dot theta l m g
eq1=tau==In*theta_dbl_dot;
eq2=-tau--m*g*sin(theta)*l;
sol=solve(eq1,eq2,theta_dbl_dot,m,g,l,theta,In,tau);
theta_dbl_dot_=(sol.theta_dbl_dot)

I believe the answer should be theta_dbl_dot = -(g/l)*sin(theta)

What am I doing wrong? Also, is there a way to linearize the equation or solve the characteristic equation after solving for the non-linear form? I have values for m, g, l, In, and theta.