Answer is QM = 18
You can find the length of QM by using Pythagorean theorem. See the attachment.
It is a quadrilateral with 4 equal congruent sides.
If SP = 30, so do the other 3 sides indicated by the tick marks on all 4 sides.
We will call side RQ = the hypotenuse
or side “c” = 30
We know RM is leg “b” = 24
Side “a” is our unknown QM
Our formula is
a^2 + b^2 = c^2
a^2 + 24^2 = 30^2
a^2 + 576 = 900
a^2 = 900 - 576
a^2 = 324
Take square root of both sides to solve a
a = 18
QM = 18