1.8 - (2 3/5-(x-1.2) )=3

2 3/5переводим=2.6

1.8-(2.6-(х-1.2))=3
-2.6(х-1.2)=3-1.8=1.2
х-1.2=1.2+2.6=3.8
х=3.8+1.2=5