Question 180718
Equation #1: 

"The book cost of Hiro’s biology book was $13.00 more than the cost of his history book" ----> {{{b=h+13}}}



Equation #2:

"The cost of his English was $27.00 less than the cost of his history book" ---> {{{e=h-27}}}



You have 3 variables but only 2 equations. So you'll get infinite number of solutions. You need 3 equations.