IF statement help

I have the two functions below

Irribarren number for random waves, ξP 24.67

Significant wave run-up, RS 133.23

I want when ξP > 9 then the value of RS is represented as zero.

But also when 0 < ξP < 2 RS = Hs * 1.35ξP And when 2 < ξP < 9 RS = Hs * (3.00 - 0.15ξP) Any formulas and help would be grateful