#### Answer

m=5

#### Work Step by Step

We need to get the m alone. Thus, we cancel out the addition by subtracting 4 on both sides. This leaves us with the equation $4.2m=21$. In order to cancel out multiplication, we use division, so we divide both sides of the equation by 4.2 to get that m=5. We plug 5 in for m and confirm that our answer is, indeed, correct.