In[62]:= eq = x^2 + Sin[4*a] == 3 - Derivative[1][y][t] Out[62]= x^2 + Sin[4*a] == 3 - Derivative[1][y][t] In[63]:= lhs = eq /. (lhs_) == (rhs_) -> lhs Out[63]= x^2 + Sin[4*a] In[64]:= rhs = eq /. (lhs_) == (rhs_) -> rhs Out[64]= 3 - Derivative[1][y][t]