Question 617687
Let A be the price of the algebra book
Let B be the price of the biology book. 
You are told
{{{A + B = 219}}}
{{{B = 2A + 15}}} 
substitute the value for B in this second equation into the first one
{{{A + (2A+15) = 219}}} now simplify and solve for A
{{{3A + 15 = 219}}}
{{{3A = 204}}}
{{{A = 204/3}}}
{{{A = 68}}}
you have A, now solve for B using the second equation above