b, 2 + 4 + 6 +.....+2x =210
=>2*(1+2+3+....+x) =210
=> 1+2+3+....+x = 105
=> (x+1)*x : 2 = 105
=> (x+1)*x = 210
=> (x+1)*x = 15*14
=>x=14
c) [(x - 300)^2 - 280 ] * 2 = 19440
=> (x - 300)^2 - 280 = 9720
=> (x - 300)^2 = 10000 =1002
=> x - 300 =100
=> x=400
a, 3x-1+ 5* 3x-1 = 162
3x-1 * (5 + 1) = 162
3x-1 * 6 = 162
3x-1 = 27 = 33
=> x-1=3
=> x=4