4y2 +5y_21=0

Answer 1

do you mean
4 y^2 + 5 y - 21 =0 ??
if so
y = [ -5 +/- sqrt(25 + 336) ] / 8
y = [ -5 +/- 19 ] / 8
y = -24/8 or 14/8 which is -3 or +7/4

Answer 2

(4y+7)(y-3) = 0