How do you solve 3×6+16-4=5

Arley is building a model of a city map. in one part of the city, three roads form a right triangle, which harley draws as triangle abc, with the following measures: m∠b=90° and m∠a=30°. in his scale model, the hypotenuse of triangle abc, ac¯¯¯¯¯¯¯¯, has a length of 817−−√ cm. what is the value of a (the length of bc¯¯¯¯¯¯¯¯)?

What is the product? a-3/15a × 5/a-3

Idon't get undoing if its pass adding and subtracting so can someone ? x-2 over 5 = 18