#1
(3a+b)(2a-b)-6(a-b)²=6a²-3ab+2ab-b²-6(a²-2ab+b²)=6a²-ab-b²-6a²+12ab-6b²=-7b²+11ab
#2
{3x-2y=14
{2x+y=7|*2
+{3x-2y=14
+{4x+2y=14
7x=28|:7
x=4
2*4+y=7
y=7-8
y=-1
#3
(x-2)²-2(x+1)²=(3-x)(3+x)
x²-4x+4-2(x²+2x+1)=9-x²
x²-4x+4-2x²-4x-2=9-x²
x²-4x-2x²-4x+x²=9+2-4
-8x=7|:(-8)
x=-7/8
#4
S-x
t1-y
{x+1=3y
{x=5(3-y)
-{x-3y=-1
-{x+5y=15
-8y=-16|:(-8)
y=2
x+1=2*3
x=6-1
x=5
S=5+5+1=11(км)