<span>6y-3(y-1)=4+5y </span>
<span>6у-3у+3=4+5у</span>
<span>3у-5у=4-3</span>
<span>-2у=1</span>
<span>у=-1/2=-0,5</span>
a1=-5
S7=(2a1+6d)/2*7=28
7(a1+3d)=28
a1+3d=4
d=(4-a1)/3
d=(4+5)/3=3
a2=a1+d=-5+3=-2