2x + x = 18
3x = 18
x = 6
7x + 3 = 30 - 2x
7x + 2 x = 30 - 3
9x = 27
x= 3
7 - 2x = 3x - 18
7 + 18 = 2x + 3x
25 = 5x
x = 5
..............................
1)(1/9)x²z²-a²=(xz/3+a)(xz/3-a)
2)y²-163=(y+√163)(y-√163)
3)100-9b²=10²-(3b)²=(10+3b)(10-3b)
4)c²d²-81x²=(cd)²-(9x)²=(cd+9x)(cd-9x)
5)4a²/25-9b²c²=(2a/5)²-(3bc)²=(0,4a)²-(3bc)²=(0,4a+3bc)(0,4a-3bc)
(? 2)y²-169=y²-13²=(y+13)(y-13))