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The missing number is 21.

Solution:
Every line is a calculation. The numbers to use in the calculation is the
number of corners in the symbols. The number written on each line is equal to
the calculation plus 1.

I.E.

Triangle +
Square = 8
is
3 + 4 = 7 + 1 = 8

So, to find the missing number: 5 × 4 = 20 + 1 = 21