2) (6+y)^2-81=0

(6+y)^2-9^2=0

(6+y-9)(6+y+9)=0

(y-3)(15+y)=0

y=3 или y=-15

4)25- (13+t)^2=0

5^2-(13+t)^2=0

(5-13-t)(5+13+t)=0

(-8-t)(18+t)=0

t=-8 или t=-18