Question 1090081
question #1:

{{{ y = mx + b}}} (for {{{m}}}) .......add {{{-b}}} to both sides


{{{ y-b = mx + b-b}}}


{{{ y-b = mx }}}...both sides divide by {{{x}}}

{{{ (y-b)/x = m(cross(x))/cross(x) }}}

{{{ (y-b)/x = m }}}


and question #2: 


{{{P = 2(l + w)}}} ...(for{{{ w}}}) ...both sides divide by {{{2}}}


{{{P/2 = cross(2)(l + w)/cross(2)}}}


{{{P/2 = l + w}}}....add {{{-l}}} to both sides


{{{P/2 -l= l-l + w}}}


{{{P/2 -l=w}}}