Answer
$b_s=\frac{g_c-y_t}{r_p}$
Work Step by Step
Let $r_p=Red Pentagon$
$b_s=Blue Square$
$y_t=Yellow Triangle$
$g_c=Green Circle$
$r_p*b_s+y_t=g_c$
Solve for $b_s$.
$r_p*b_s=g_c-y_t$
$b_s=\frac{g_c-y_t}{r_p}$
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