Question

A textbook store sold a combined total of 319 chemistry and history textbooks in a week. The number of history textbooks sold was 89 less than the number of chemistry textbooks sold. How many textbooks of each type were sold?

Answer 1

H = history textbooks 
c = chemistry textbooks

For this you'll have to do a system of equations:
$\left \{ {{h + c=319} \atop {h=c-89}} \right. \\ (c-89) + c = 319 \\ 2c - 89 = 319 \\ 2c - 89 +89 = 319 + 89 \\ 2c = 408 \\ 2c/2 = 408/2 \\ c = 204 \\ \\ h = c-89 \\ h=204-89 \\ h=115$

115 history textbooks and 204 chemistry textbooks were sold.
Hope this helps!