P varies jointly as r and m^2 means:

P = C*r*m^2, where C is constant

When r=5 and m=3ˏ P=90 means

90 = C*5*3^2

by solving we find:

C = 2

click here to see the step by step solution of the equation:

P = 2*r*m^2

when r = 2 and m = 5 for P we find:

P = 2*2*5^2

P = 100

when r is 2 and m is 5 for P we find: P is 100.