-61=-7(3b+1)-6b

-61  =  -7 ( 3b + 1 ) - 6b

Use the distribuitve property on -7 ( 3b + 1)   to get    -7(3b)  +  -7(1)    to get    -21b - 7

-61  =  -21b - 7 - 6b  

Combine the  -21b - 6 b  to get  -27b

-61  =  -27b - 7

Add 7 to both sides:

-61 + 7  =  -27b - 7 + 7

-54  =  -27b

Divide both sides by -27   -->  -54 / -27  =  2

2  =  b

$$-{\mathtt{61}} = {\mathtt{\,-\,}}{\mathtt{7}}{\mathtt{\,\times\,}}\left({\mathtt{3}}{\mathtt{\,\times\,}}{\mathtt{b}}{\mathtt{\,\small\textbf+\,}}{\mathtt{1}}\right){\mathtt{\,-\,}}{\mathtt{6}}{\mathtt{\,\times\,}}{\mathtt{b}} \Rightarrow {\mathtt{b}} = {\mathtt{2}}$$