Find PQ for P(-3,5) and Q(-5,8)

Let x = the strawberry wafers and y = the chocolate ones
x+y = 22   (the number of wafers they bought it 22)
3x + y = $30   (the cost of the $3 wafers and $1 wafers = $30)
 
if i add 8 to the first equation they will both be worth 30.   (x+y+8=30)
since they are now both equal to the same thing (30) i will set them equal to each other.
3x+y = x+y+8   now lets get the like terms on the same side by subtracting x from
                        3x and y from y
2x = 8   (notice the y's are gone since y -y =0)
divide both sides by 2 and get x= 4
so 4 packets of the $3 wafers.     and since x+y = 22   4 +? =22   (answer 18)
final answer: 4 packets of the $3 wafers and 18 packets of $1 wafers
check: let try it in our other equation   3x + y = $30 (remember x=4, y=18)
3(4) +18 = $30
12 + 18 = 30
        30   = 30