In: Advanced Math
Solve the following system of linear equations:
−p + q + 2r = 8 (1)
p + q + 2r = 0 (2)
2q − 2r = 2 (3).
The sum of the values of p, q and r of the solution is

back substitution means
starting from last equation and finding the variable's value then
using that variable's value in 2nd equation and then solving the
first equation for the first variable.