a ( 3 ; 1 )

b ( 1 ;  2 )

hb ( 1h ; 2h )

a+hb ( 3 + 1h ; 1 + 2h )

Скалярное = 0

( a + hb ) * b = ( 3+1h )*1+( 1+2h )*2 = 3+1h+2+4h=5h+5

5h+5=0

5h=-5

h=-1