Question

Im so confused somebody please help and explain.

Answer 1

$\\ \sf\longmapsto 3x+4x-1=90$

$\\ \sf\longmapsto 7x-1=90$

$\\ \sf\longmapsto 7x=90+1=91$

$\\ \sf\longmapsto x=\dfrac{91}{7}$

$\\ \sf\longmapsto x=13$