2.8m+6.6= --21.4?

Take the +6.6 over to the other side (taking it over the (= sign) so reverse the sign)

2.8m = -21.4 - 6.6
2.8m = -28

Divide the 2.8/2.8 from m to have m on its own, do the same to the other side (-28/2.8)

m = -10

restrictions are that m is negative!!

2.8 m= -21.4-6.6=-28.
m= -28/2.8= - 10.
Ans. m= -10.

2.8m +6.6=-21.4
or 2.8=-21.4-6.6=28
or m=-28/2.8= -10
ans m= -10

2.8 m = 28
m = 10

2.8m + 6.6 = -21.4
2.8m = -21.4 - 6.6
2.8m = -28
m = -28/2.8
m = 10