The answer is 91
' W9 S+ P* H8 G' \8 ~1 row 3x4+1x3=15* r# q3 e4 {3 h/ @6 V4 j0 D
2 row 4x5+2x4=28 f% Z* F, P* _+ |/ }6 E/ `3 c/ f% C
3 row 5x6+3x5=45tvb now,tvbnow,bttvb' H% O6 C1 z: D! _" y
4 row 6x7+4x6=66tvb now,tvbnow,bttvb; A# F8 }2 T! k) H2 T
5 row 7x8+5x7=91  |