I'm getting a little rusty at this sort of thing myself.

The formula for gravitational acceleration g at the surface of a planet is

g = GM/R**2 (the test mass is irrelevant because it is assumed to be much, much lighter than the planet)

where

g is the gravitational acceleration in m/s**2
G is the constant of proportionality that makes it all come out in the right units
M is the mass of the planet in kg
R is the radius of the planet in meters

Let's normalize the data by redefining the above variables to 1

1.5 is the gravitational acceleration in earth g's
5 is the planet mass in earth masses
r is the unknown radius we are trying to determine in earth radii
(Normalizing will require a new value of G, so let's call it Gprime, although we don't need to know it)

The equation now becomes

1.5 = Gprime5/r**2 and reordering terms,

r**2 = (5Gprime)/1.5 since M and g are now both normalized to 1.0

therefore

r**2 = (3.33333...)Gprime

r = SQRT (3.333...) Gprime drops out as all the units cancel out

r = 1.8257 earth radii

This stands up to the eyeball check: if the planet massed 4 times as much it would have to have twice the radius to give the same surface gravity, due to inverse square law, so my result seems to be at least in the ballpark.