Es. N. 91

91)

 $9a^2-(+3a^2)$ = 

= $9a^2-3a^2$ =

= $6a^2$

$-15x-(-20x)$ =

= $-15x+20x$ =

= $+5x$

$-8a^3b-(-5a^3b)$ =

= $-8a^3b+5a^3b$ =

= $-3a^3b$