x=-7 AB=-72
steps
since m is a midpoint, AM and MB are congruent so you set the two equations equal to each other, 4x-9=5x-3/3( is this a typo? bc 3/3 is 1)
subtract 4x from both sides m, -8=x-1
add 1 to both sides
x=-7
then plug -7 in for x 4(-7)-8=-36
since the two segments are congruent u do -36 x 2= -72
