How can i find equivalent resistance?

When two resistors are in series, you add the elements together. When two elements are in parallel you multiply them and divide that by the sum. ie: parallel elements 7 ohms and 8 ohms (8*7)/(8+7).

If there are more than 2 in parallel however, just fill in this:

1/Req=1/R1 + 1/R2 + 1/RN

Req is the final answer of the equivalent resistance.
Rn means you keep adding the reciprocal of the other resistors in parallel.