We solve the as follows:
1. Solve proportion 12/15 = 18/b
( 12/15 = 18/b ) b
( 12b/15 = 18 ) 15/12
b = 18(15/12)
b = 45/2
2. Solve equations
A. -x + 4 = x +6
-x + 4 - x = x +6 - x
-2x + 4 - 4 = 6 - 4
-2x = 2
x = -1
B. 5n + 7 =7(n+1) -2n
5n + 7 = 7n + 7 - 2n
5n -7n + 2n = 7 - 7
0 = 0
C. -4(p+2) + 8 = 2(p-1) - 7p + 15
-4p - 8 + 8 = 2p - 2 - 7p + 15
-4p + 7p - 2p = -2 + 15 + 8 - 8
p =13
3. Solve a/b x - c = 0 for x
a/b x - c = 0
( a/b x = c ) b/a
x = bc / a