mathematica tag protected 问题? 30

怎么解决这个问题？ 看图我要怎么输入才可以解方程

In[11]:= Solve[{3 x + 2 y = 15, 3 x - 3 y = 12}, {x, y}]

During evaluation of In[11]:= Set::write: Tag Plus in 3 x+2 y is Protected. >>

During evaluation of In[11]:= Set::write: Tag Plus in 3 x-3 y is Protected. >>

During evaluation of In[11]:= Solve::naqs: 15&&12 is not a quantified system of equations and inequalities. >>

Out[11]= Solve[{15, 12}, {x, y}]