Please study this carefully. Look up /. in the documentation and understand it.
In[1]:= Solve[{ x + y == 5 , x - y == 3 }, {x ,y} ]
Out[1]= {{x->4,y->1}}
In[2]:= {x,y}/.Solve[{ x + y == 5 , x - y == 3 }, {x ,y} ]
Out[2]= {{4,1}}
In[3]:= {{a,b}}={x,y}/.Solve[{ x + y == 5 , x - y == 3 }, {x ,y} ]
Out[3]= {{4,1}}
In[4]:= a
Out[4]= 4
In[5]:= b
Out[5]= 1
In[6]:= 2*a-3*b
Out[6]= 5
