we use Snell's law here:

it states that:

well, more generally, it states that , but let's not worry about that

where is the refractive index of the initial material (in this case, that of the air)

is the refractive index of the final material (in this case, that of the plastic)

is the angle of incidence

is the angle of refraction

you are expected to know that the refractive index of air, , just use 1, that's the regular convention

the only unknown here is (which is what we are asked to find obviously), so plug everything else in the formula and solve for it, we have:

i leave the rest to you