Hello, thank you for taking the time to post your question!
The key on this type of question is to find a way to relate the variables to one another! On this one we have
B = 2A + 15 since the biology book costs $15 more than twice the cost of the algebra book
B + A = 129 since the total cost of the two books is $129
From there then I would substitute the first equation into the second equation and then solve algebraically.
That means taking
(2A + 15) + A = 129
3A + 15 = 129
3A = 114
A = 38, meaning that
B = 2(38) + 15 = 76 + 15 = 91
The cost of the biology book is $91
I hope that helps you get moving in a better direction on this type of question! Feel free to reach out if you have any additional questions beyond that :)
