х-5(х-4)=6х+5
х-5х+20=6х+5
х-5х-6х=5-20
-10х=-15
х=-15/(-10)
х=1,5
x-5x-20=6x+5
x-5x-6x=5-20
-10x=-15
-x=1,5
x=-1.5
a(3) = a(1) + 2d = 25