Anybody good at 8th grade math that can help me

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

The value, v, of kalani’s stock investments over a time period, x, can be determined using the equation v=750(0.80)^-x. what is the rate of increase or decrease associated with this account? 20% decrease 20% increase 25% decrease 25% increase

What is the definition of a surd.give an example.